QMainWindow with many QWidgets: background application comes visible after changing focus on Split view

Preconditions:

Application includes QMainWindow with many QWidgets: QLineEdit (top of the screen), QPlainTextEdit, QSpinBox, QDateTimeEdit (bottom of the screen).
View is on portrait orientation.
Position of the QWidgets: bottom of the screen or fullscreen.

Test steps:
1. Tap the QDateTimeEdit on bottom of the screen.
-> split view is opened on portrait orientation.
2. Change the focus to QSpinBox.

Expected result:
QSpinBox is focused on split view. No background application is visible.

Actual result:
QSpinBox is focused on split view. Background application comes visible to split view screen area.