Modeless dialog moves and gets smaller when its parent is moved

A QDialog that is created with a QMainWindow as parent will move and resizes until minimum when the QMainWindow is moved. ( See movie )

The DialogTest project just allocates a QDialog : m_dialog = new ModelessDialog(this);
and upon pressing the test button calls m_dialog->show();
Then moving the main window will resize the dialog.

Somehow this bug affects all Versions since Qt 6.5.3 ( 6.5.2 is ok )

When de parent is not set, the dialog will stay as expected.