Wrong Hi-Dpi Scaling with X on Kubuntu

I had issues with Kubuntu and my 4k monitor, is it never saved the display scale factor correctly.
Thus, I set the X config and .Xresources manually to 192x192 dpi:
https://www.girialam.com/2016/02/kubuntu-and-hidpi-screen/

When I started QtCreator, font and window scale was twice as large as expected. The device pixel ratio is set to 2.
I tested the env variables QT_DEVICE_PIXEL_RATIO and QT_AUTO_SCREEN_SCALE_FACTOR in all combinations (screen shot attached(. The only correct output was achieved with (QT_DEVICE_PIXEL_RATIO=1 (but it was expected to be 2!) and QT_AUTO_SCREEN_SCALE_FACTOR=1).

QML and QWidget windows seem to scale differently.
This might be related to https://bugreports.qt.io/browse/QTCREATORBUG-16828