QWidget::createWindowContainer has transparent area.

When using 

QWindow::fromWinId(wid);
QWidget::createWindowContainer(window, container, Qt::Widget);

to embed a PySide2/6 window into a C++ window, would create a strange transparent area. Like this,

The parent widget has a red background with auto fill background on. The window container widget does not have any background. The embedded window does not fullfill the container widget as expected. C++ and Python source code attached.