diff --git a/CHANGELOG.md b/CHANGELOG.md
index faad379..aff30d3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,7 @@
 - Fix a `KeyAlreadyPresent` error when parsing or accessing an out-of-order table whose array-of-tables elements are split across the table's parts. ([#505](https://github.com/python-poetry/tomlkit/issues/505))
 - Out-of-order value-vs-table and dotted-key-vs-table redefinitions are now rejected at parse time instead of being silently accepted or raising only on access. The parser also detects when a non-dotted key is a prefix of an existing dotted key, matching the stdlib `tomllib` behaviour. ([#523](https://github.com/python-poetry/tomlkit/issues/523))
 - Reject tables inserted into inline tables instead of serializing invalid TOML. ([#531](https://github.com/python-poetry/tomlkit/issues/531))
+- Fix a new top-level scalar being captured by a table rendered from a dotted key: appending a scalar after a dotted-key entry (e.g. `a.b = 1`) whose table had gained a `[a.c]`-style child placed the scalar inside that table's scope, silently re-nesting it on round-trip. Scalars now move before such an entry, like they do before regular tables. ([#543](https://github.com/python-poetry/tomlkit/issues/543))
 
 ## [0.15.0] - 2026-05-10
 
diff --git a/tests/test_toml_document.py b/tests/test_toml_document.py
index 0ee5ed8..ca303e8 100644
--- a/tests/test_toml_document.py
+++ b/tests/test_toml_document.py
@@ -1560,3 +1560,30 @@ def test_appending_to_super_table() -> None:
 """
 
     assert doc.as_string() == expected
+
+
+def test_scalar_is_not_captured_by_table_rendered_from_dotted_key() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/543
+    # A dotted-key super table renders inline (`a.b = 1`) until a child that
+    # renders a `[table]` header is added; a scalar appended after that must
+    # not land inside the table's scope.
+    doc = parse("a.b = 1\n")
+    doc["a"]["c"] = {}
+    doc["z"] = 2
+
+    expected = """\
+z = 2
+
+a.b = 1
+
+[a.c]
+"""
+
+    assert doc.as_string() == expected
+    assert parse(doc.as_string()) == {"z": 2, "a": {"b": 1, "c": {}}}
+
+    # A purely inline dotted key still gets the scalar appended after it.
+    doc = parse("a.b = 1\n")
+    doc["z"] = 2
+
+    assert doc.as_string() == "a.b = 1\nz = 2\n"
diff --git a/tomlkit/container.py b/tomlkit/container.py
index 040bde7..6c80ad4 100644
--- a/tomlkit/container.py
+++ b/tomlkit/container.py
@@ -161,9 +161,33 @@ def _get_last_index_before_table(self) -> int:
 
             if isinstance(v, (Table, AoT)) and k is not None and not k.is_dotted():
                 break
+
+            if (
+                isinstance(v, Table)
+                and k is not None
+                and k.is_dotted()
+                and self._renders_table_header(v)
+            ):
+                # A dotted-key super table renders inline (`a.b = 1`) only as
+                # long as none of its children render a `[table]` header; once
+                # one does, anything appended after it would land inside that
+                # table's scope.
+                break
             last_index = i
         return last_index + 1
 
+    def _renders_table_header(self, table: Table) -> bool:
+        for k, v in table.value.body:
+            if isinstance(v, AoT):
+                return True
+            if isinstance(v, Table):
+                if k is not None and k.is_dotted() and v.is_super_table():
+                    if self._renders_table_header(v):
+                        return True
+                else:
+                    return True
+        return False
+
     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):