A quick update on this.
Someone got asigned to it, and theres a workaround, in the in the broader sense of the word.
There's the Qt::WindowStates flag Qt::MaximizeUsingFullscreenGeometryHint that you can set:
now the App fills the whole screen in Portrait-and Landscapemode.
Tor Arne Vestbø, to whom the ticket was asigned to, says an automatic content margin is in the making, but untill than one has to adjust the UI oneself.
Tor Arne Vestbø said in QTBUG-64574:
If you set the Qt::MaximizeUsingFullscreenGeometryHint window flag, the window will also cover the area under the statusbar, etc. But we don't have any APIs to automatically add margins for the inner content, the stuff you don't want under the statusbar, so for now you have to do that manually. I'm working on the latter as we speak.
I had no idea, that that window flag existed, or was not aktivated by default - on mobile apps.
I can work with that. The ticket is still open but my problems are solved. I'll close this topic.