QVideoWidget always draws on top of QStackedWidget

I created a QStackedWidget and set its stackingMode to StackAll by

auto layout = qobject_cast<QStackedLayout*>(ui->preview->layout());
layout->setStackingMode(QStackedLayout::StackingMode::StackAll);

But the QVideoWidget in lower page always draws on top of higher page.

The QVideoWidget is promoted from QWidget.

See the screenshots, yellow "1"s are covered by only QVideoWidget.