diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3737b80e..38f8028f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fix mutations that make a table render a `[header]` while a bare or dotted key that is not inside it still follows at the same level, so the header swallows that key when the output is parsed again. This includes assigning a table over a child of an out-of-order dotted-key table (`doc["a"]["b"] = {...}` over `a.b`/`a.c`/`a.d`), doing so inside a regular table or array-of-tables element, promoting children in multiple super tables, and adding a table to a super table (`a.b = 1` then `doc["a"]["new"] = {...}`). Rendering now keeps relative inline keys before absolute child headers at every scope, preserves enclosing table and array-of-tables prefixes, and moves leading comment and whitespace trivia with the key it documents. ([#556](https://github.com/python-poetry/tomlkit/issues/556))
+
 ## [0.15.1] - 2026-07-17
 
 ### Changed
diff --git a/tests/test_toml_document.py b/tests/test_toml_document.py
index 7ba61f5e..5807c2af 100644
--- a/tests/test_toml_document.py
+++ b/tests/test_toml_document.py
@@ -1210,6 +1210,240 @@ def test_replace_value_with_table_keeps_following_dotted_sibling() -> None:
     assert parse(doc.as_string()) == {"c": {"d": 2}, "x": {}}
 
 
+def test_replace_out_of_order_dotted_child_with_table_keeps_following_siblings() -> (
+    None
+):
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # ``a.b``/``a.c``/``a.d`` parse as out-of-order fragments of super table
+    # ``a`` (so ``doc["a"]`` is an OutOfOrderTableProxy). Replacing a child with
+    # a real table promotes that fragment to a ``[a.b]`` header, which must move
+    # past the following inline siblings instead of swallowing them.
+    content = """a.b = 1
+a.c = 2
+a.d = 3
+"""
+    doc = parse(content)
+    doc["a"]["b"] = {"x": 9}
+    assert (
+        doc.as_string()
+        == """a.c = 2
+a.d = 3
+
+[a.b]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"c": 2, "d": 3, "b": {"x": 9}}}
+    assert parse(doc.as_string()).unwrap() == doc.unwrap()
+
+
+def test_replace_out_of_order_dotted_child_keeps_following_other_group() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # The promoted header must also clear a following dotted key of a different
+    # top-level group, not only same-group siblings.
+    content = """a.b = 1
+a.c = 2
+q.e = 3
+"""
+    doc = parse(content)
+    doc["a"]["b"] = {"x": 9}
+    assert (
+        doc.as_string()
+        == """a.c = 2
+q.e = 3
+
+[a.b]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"c": 2, "b": {"x": 9}}, "q": {"e": 3}}
+    assert parse(doc.as_string()).unwrap() == doc.unwrap()
+
+
+def test_replace_out_of_order_dotted_child_with_empty_table_keeps_sibling() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    content = """a.b = 1
+a.c = 2
+"""
+    doc = parse(content)
+    doc["a"]["b"] = {}
+    assert (
+        doc.as_string()
+        == """a.c = 2
+
+[a.b]
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"c": 2, "b": {}}}
+
+
+def test_replace_out_of_order_dotted_child_with_aot_keeps_sibling() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    content = """a.b = 1
+a.c = 2
+"""
+    doc = parse(content)
+    arr = tomlkit.aot()
+    tbl = tomlkit.table()
+    tbl["x"] = 9
+    arr.append(tbl)
+    doc["a"]["b"] = arr
+    assert (
+        doc.as_string()
+        == """a.c = 2
+
+[[a.b]]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"c": 2, "b": [{"x": 9}]}}
+
+
+def test_replace_last_out_of_order_dotted_child_does_not_move() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # With nothing rendering inline after it, the promoted header stays in place.
+    content = """a.b = 1
+a.c = 2
+"""
+    doc = parse(content)
+    doc["a"]["c"] = {"x": 9}
+    assert (
+        doc.as_string()
+        == """a.b = 1
+
+[a.c]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"b": 1, "c": {"x": 9}}}
+
+
+def test_replace_nested_out_of_order_dotted_child_keeps_siblings() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # The promotion can happen a level down: a.b.c / a.b.d / a.e parse as
+    # out-of-order fragments and doc["a"]["b"]["c"] = {...} promotes c to
+    # [a.b.c]. The captured siblings live in other top-level a fragments, so the
+    # fix has to normalise the document body, not just the container mutated.
+    content = """a.b.c = 1
+a.b.d = 2
+a.e = 3
+"""
+    doc = parse(content)
+    doc["a"]["b"]["c"] = {"x": 9}
+    assert (
+        doc.as_string()
+        == """a.b.d = 2
+a.e = 3
+
+[a.b.c]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {"a": {"b": {"c": {"x": 9}, "d": 2}, "e": 3}}
+    assert parse(doc.as_string()).unwrap() == doc.unwrap()
+
+
+def test_replace_deeply_nested_out_of_order_dotted_child_keeps_siblings() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # Same at one more level of dotting, to confirm the fix is depth-agnostic.
+    content = """a.b.c.d = 1
+a.b.c.e = 2
+a.f = 3
+"""
+    doc = parse(content)
+    doc["a"]["b"]["c"]["d"] = {"x": 9}
+    assert (
+        doc.as_string()
+        == """a.b.c.e = 2
+a.f = 3
+
+[a.b.c.d]
+x = 9
+"""
+    )
+    assert parse(doc.as_string()) == {
+        "a": {"b": {"c": {"d": {"x": 9}, "e": 2}}, "f": 3}
+    }
+
+
+def test_add_table_to_super_table_keeps_following_top_level_value() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/556
+    # Adding a table to a super table makes it render a trailing [a.new] header
+    # even though it is not out of order; a following top-level value must move
+    # before that header instead of being swallowed by it.
+    content = """a.b = 1
+z = 2
+"""
+    doc = parse(content)
+    doc["a"]["new"] = {"x": 1}
+    assert (
+        doc.as_string()
+        == """z = 2
+a.b = 1
+
+[a.new]
+x = 1
+"""
+    )
+    assert parse(doc.as_string()) == {"z": 2, "a": {"b": 1, "new": {"x": 1}}}
+    assert parse(doc.as_string()).unwrap() == doc.unwrap()
+
+
+def test_add_table_to_super_table_without_trailing_newline_keeps_separator() -> None:
+    doc = parse("a.b=1\nz=2")
+    doc["a"]["new"] = {"x": 1}
+
+    output = doc.as_string()
+
+    assert output == "z=2\na.b=1\n\n[a.new]\nx = 1\n"
+    assert parse(output).unwrap() == doc.unwrap()
+
+
+def test_promoted_children_of_multiple_super_tables_preserve_roundtrip() -> None:
+    doc = parse("a.x=1\nq.x=2\nr=3\n")
+    doc["a"]["new"] = {"v": 1}
+    doc["q"]["new"] = {"v": 2}
+
+    output = doc.as_string()
+
+    assert output.index("r=3") < output.index("a.x=1")
+    assert output.index("a.x=1") < output.index("q.x=2")
+    assert output.index("q.x=2") < output.index("[a.new]")
+    assert output.index("[a.new]") < output.index("[q.new]")
+    assert parse(output).unwrap() == doc.unwrap()
+
+
+def test_promoted_child_inside_table_keeps_parent_prefix() -> None:
+    doc = parse("[outer]\np.b = 1\np.c = 2\n")
+    doc["outer"]["p"]["b"] = {"x": 1}
+
+    output = doc.as_string()
+
+    assert "[outer.p.b]" in output
+    assert parse(output).unwrap() == doc.unwrap()
+
+
+def test_promoted_child_inside_aot_keeps_parent_prefix_and_siblings() -> None:
+    doc = parse("[[p]]\na.b = 1\na.c = 2\nz = 3\n")
+    doc["p"][0]["a"]["b"] = {"x": 1}
+
+    output = doc.as_string()
+
+    assert "[p.a.b]" in output
+    assert parse(output).unwrap() == doc.unwrap()
+
+
+def test_promoted_child_moves_leading_comment_with_inline_sibling() -> None:
+    doc = parse("a.b = 1\n# Documentation for a.c\na.c = 2\n[z]\nq = 3\n")
+    doc["a"]["b"] = {"x": 9}
+
+    output = doc.as_string()
+
+    assert output.index("# Documentation for a.c") < output.index("a.c = 2")
+    assert output.index("a.c = 2") < output.index("[a.b]")
+    assert parse(output).unwrap() == doc.unwrap()
+
+
 def test_replace_with_comment() -> None:
     content = 'a = "1"'
     doc = parse(content)
diff --git a/tomlkit/container.py b/tomlkit/container.py
index 8ff30d98..6b2105ca 100644
--- a/tomlkit/container.py
+++ b/tomlkit/container.py
@@ -188,6 +188,180 @@ def _renders_table_header(self, table: Table) -> bool:
                     return True
         return False
 
+    @staticmethod
+    def _append_header(rendered: str, header: str) -> str:
+        if not header:
+            return rendered
+
+        # Match the separator rule previously repeated in each rendering loop.
+        # A header's own trivia can already contain the required newline.
+        before_header = header.partition("[")[0]
+        if (
+            rendered.strip(" ")
+            and not rendered.strip(" ").endswith("\n")
+            and "\n" not in before_header
+        ):
+            rendered += "\n"
+        return rendered + header
+
+    @staticmethod
+    def _append_inline(rendered: str, inline: str) -> str:
+        if not inline:
+            return rendered
+
+        if (
+            rendered.strip(" ")
+            and not rendered.rstrip(" ").endswith(("\n", "\r"))
+            and not inline.lstrip(" ").startswith(("\n", "\r"))
+        ):
+            rendered += "\n"
+        return rendered + inline
+
+    @staticmethod
+    def _join_key(prefix: str | None, key: Key) -> str:
+        rendered = key.as_string()
+        return f"{prefix}.{rendered}" if prefix is not None else rendered
+
+    def _table_emits_own_header(self, key: Key, table: Table) -> bool:
+        return (
+            not table.is_super_table()
+            or (
+                any(
+                    not isinstance(v, (Table, AoT, Whitespace, Null))
+                    for _, v in table.value.body
+                )
+                and not key.is_dotted()
+            )
+            or (
+                any(
+                    k is not None and k.is_dotted()
+                    for k, v in table.value.body
+                    if isinstance(v, Table)
+                )
+                and not key.is_dotted()
+            )
+        )
+
+    def _render_body_parts(
+        self,
+        *,
+        header_prefix: str | None = None,
+        inline_prefix: str | None = None,
+    ) -> tuple[str, str]:
+        """Render a container as stable inline and header partitions.
+
+        TOML keys at one scope must precede child table headers. A mutated super
+        table can contribute to both partitions, so ordering whole body entries
+        is insufficient when two such tables are interleaved. Leading comments
+        and whitespace travel with the keyed entry they describe.
+        """
+
+        pure_inline = ""
+        mixed_inline = ""
+        headers = ""
+        pending_trivia = ""
+
+        for key, item in self._body:
+            if key is None:
+                pending_trivia += self._render_simple_item(key, item)
+                continue
+
+            item_inline, item_headers = self._render_item_parts(
+                key,
+                item,
+                header_prefix=header_prefix,
+                inline_prefix=inline_prefix,
+            )
+            if item_inline and item_headers:
+                mixed_inline += pending_trivia + item_inline
+                pending_trivia = ""
+            elif item_inline:
+                pure_inline += pending_trivia + item_inline
+                pending_trivia = ""
+            elif item_headers:
+                item_headers = pending_trivia + item_headers
+                pending_trivia = ""
+
+            if item_headers:
+                headers = self._append_header(headers, item_headers)
+
+        if pending_trivia:
+            if headers:
+                headers += pending_trivia
+            else:
+                pure_inline += pending_trivia
+
+        return self._append_inline(pure_inline, mixed_inline), headers
+
+    def _render_item_parts(
+        self,
+        key: Key,
+        item: Item,
+        *,
+        header_prefix: str | None,
+        inline_prefix: str | None,
+    ) -> tuple[str, str]:
+        if isinstance(item, Table):
+            return self._render_table_parts(
+                key,
+                item,
+                header_prefix=header_prefix,
+                inline_prefix=inline_prefix,
+            )
+        if isinstance(item, AoT):
+            return "", self._render_aot(key, item, prefix=header_prefix)
+        return self._render_simple_item(key, item, prefix=inline_prefix), ""
+
+    def _render_table_parts(
+        self,
+        key: Key,
+        table: Table,
+        *,
+        header_prefix: str | None,
+        inline_prefix: str | None,
+    ) -> tuple[str, str]:
+        if table.display_name is not None:
+            full_key = table.display_name
+        else:
+            full_key = self._join_key(header_prefix, key)
+
+        if not self._table_emits_own_header(key, table):
+            relative_key = self._join_key(inline_prefix, key)
+            inline, headers = table.value._render_body_parts(
+                header_prefix=full_key,
+                inline_prefix=relative_key,
+            )
+            if table.trivia.indent == "\n":
+                if inline:
+                    inline = table.trivia.indent + inline
+                else:
+                    headers = table.trivia.indent + headers
+            return inline, headers
+
+        open_, close = "[", "]"
+        if table.is_aot_element():
+            open_, close = "[[", "]]"
+
+        newline_in_table_trivia = (
+            "\n" if "\n" not in table.trivia.trail and len(table.value) > 0 else ""
+        )
+        header = (
+            f"{table.trivia.indent}"
+            f"{open_}"
+            f"{decode(full_key)}"
+            f"{close}"
+            f"{table.trivia.comment_ws}"
+            f"{decode(table.trivia.comment)}"
+            f"{table.trivia.trail}"
+            f"{newline_in_table_trivia}"
+        )
+        body_inline, body_headers = table.value._render_body_parts(
+            header_prefix=full_key,
+            inline_prefix=None,
+        )
+        body = self._append_header(body_inline, body_headers)
+        return "", header + body
+
     def _validate_out_of_order_table(self, key: Key | None = None) -> None:
         if key is None:
             for k in list(self._out_of_order_keys):
@@ -632,113 +806,17 @@ def last_item(self) -> Item | None:
 
     def as_string(self) -> str:
         """Render as TOML string."""
-        s = ""
-        for k, v in self._body:
-            if k is not None:
-                if isinstance(v, Table):
-                    if (
-                        s.strip(" ")
-                        and not s.strip(" ").endswith("\n")
-                        and "\n" not in v.trivia.indent
-                    ):
-                        s += "\n"
-                    s += self._render_table(k, v)
-                elif isinstance(v, AoT):
-                    if (
-                        s.strip(" ")
-                        and not s.strip(" ").endswith("\n")
-                        and "\n" not in v.trivia.indent
-                    ):
-                        s += "\n"
-                    s += self._render_aot(k, v)
-                else:
-                    s += self._render_simple_item(k, v)
-            else:
-                s += self._render_simple_item(k, v)
-
-        return s
+        inline, headers = self._render_body_parts()
+        return self._append_header(inline, headers)
 
     def _render_table(self, key: Key, table: Table, prefix: str | None = None) -> str:
-        cur = ""
-
-        if table.display_name is not None:
-            _key = table.display_name
-        else:
-            _key = key.as_string()
-
-            if prefix is not None:
-                _key = prefix + "." + _key
-
-        if (
-            not table.is_super_table()
-            or (
-                any(
-                    not isinstance(v, (Table, AoT, Whitespace, Null))
-                    for _, v in table.value.body
-                )
-                and not key.is_dotted()
-            )
-            or (
-                any(
-                    k is not None and k.is_dotted()
-                    for k, v in table.value.body
-                    if isinstance(v, Table)
-                )
-                and not key.is_dotted()
-            )
-        ):
-            open_, close = "[", "]"
-            if table.is_aot_element():
-                open_, close = "[[", "]]"
-
-            newline_in_table_trivia = (
-                "\n" if "\n" not in table.trivia.trail and len(table.value) > 0 else ""
-            )
-            cur += (
-                f"{table.trivia.indent}"
-                f"{open_}"
-                f"{decode(_key)}"
-                f"{close}"
-                f"{table.trivia.comment_ws}"
-                f"{decode(table.trivia.comment)}"
-                f"{table.trivia.trail}"
-                f"{newline_in_table_trivia}"
-            )
-        elif table.trivia.indent == "\n":
-            cur += table.trivia.indent
-
-        for k, v in table.value.body:
-            if isinstance(v, Table):
-                if (
-                    cur.strip(" ")
-                    and not cur.strip(" ").endswith("\n")
-                    and "\n" not in v.trivia.indent
-                ):
-                    cur += "\n"
-                assert k is not None
-                if v.is_super_table():
-                    if k.is_dotted() and not key.is_dotted():
-                        # Dotted key inside table
-                        cur += self._render_table(k, v)
-                    else:
-                        cur += self._render_table(k, v, prefix=_key)
-                else:
-                    cur += self._render_table(k, v, prefix=_key)
-            elif isinstance(v, AoT):
-                if (
-                    cur.strip(" ")
-                    and not cur.strip(" ").endswith("\n")
-                    and "\n" not in v.trivia.indent
-                ):
-                    cur += "\n"
-                assert k is not None
-                cur += self._render_aot(k, v, prefix=_key)
-            else:
-                cur += self._render_simple_item(
-                    k, v, prefix=_key if key.is_dotted() else None
-                )
-
-        return cur
+        inline, headers = self._render_table_parts(
+            key,
+            table,
+            header_prefix=prefix,
+            inline_prefix=None,
+        )
+        return self._append_header(inline, headers)
 
     def _render_aot(self, key: Key, aot: AoT, prefix: str | None = None) -> str:
         _key = key.as_string()
@@ -767,22 +845,11 @@ def _render_aot_table(self, table: Table, prefix: str | None = None) -> str:
             f"{table.trivia.trail}"
         )
 
-        for k, v in table.value.body:
-            if isinstance(v, Table):
-                assert k is not None
-                if v.is_super_table():
-                    if k.is_dotted():
-                        # Dotted key inside table
-                        cur += self._render_table(k, v)
-                    else:
-                        cur += self._render_table(k, v, prefix=_key)
-                else:
-                    cur += self._render_table(k, v, prefix=_key)
-            elif isinstance(v, AoT):
-                assert k is not None
-                cur += self._render_aot(k, v, prefix=_key)
-            else:
-                cur += self._render_simple_item(k, v)
+        body_inline, body_headers = table.value._render_body_parts(
+            header_prefix=_key,
+            inline_prefix=None,
+        )
+        cur += self._append_header(body_inline, body_headers)
 
         return cur