Fullscreen windows not correctly displayed after playing video through QVideoWidget

After playing video through QVideoWidget, softkeys menu and sometimes status bar are visible in all previously hidden fullscreen-windows when they are shown again.

Detailed sequence of actions before problem occurs:

1. All windows (subclasses of QMainWindow) are shown in Fullscreen-Portrait-Mode (without status bar and softkey menu).

2. QVideoWidget shows video (Fullscreen-Landscape-Mode)

3. Playing of video stops correctly

4. Windows that come to front again after video-playback have softkeys menu and sometimes statusbar shown, even after calling "showFullScreen()"-function

5. After creating a new QMainWindow instance, displaying it by calling "showFullScreen()" and closing it, all other Fullscreen-Windows are displayed correctly again (without status bar and softkeys-menu).