Comment # 1 on bug 1208714 from Michael Hirmke

I forgot to mention, that the variables are set in a pre-startup script for
plasma (.config/plasma-workspace/env/scaling.sh).

I tested and read a lot of things today, including the hidpi web pages for Qt5.

export QT_FONT_DPI=192 solved the problem for most QT applications.

I learnt that GDK_SCALE and GDK_DPI_SCALE variables are no longer used in
plasma 5.27.1, instead xsettingsd should do that job, which is the "Gnome/GTK
Settings Synchronization Service" in systemsettings5. Deactivating it solved
the problem for GTK applications.
I am quite sure, none of these "solutions" are reflecting the desired state of
the system, but for the moment the system does what is necessary to use it.

Therefore I reduced the severity to normal.