Created attachment 171473 [details]
An example of the problem showing how font settings are applied in Qt6 apps but not in Qt5 apps

SUMMARY
Font settings are not applied to Qt5 apps. This includes the font family and the font size.

This happened somewhat recently, at some point after Plasma 6.0.

STEPS TO REPRODUCE
1. Set the system font to something that differs from whatever Qt5 falls back to, like Noto Sans at 11pt.
2. Open a Qt5 app, like VLC, Lokalize, or Krita
3. Open a Qt6 app like Dolphin as control

OBSERVED RESULT
Font settings are not applied to Qt5 apps, and they fall back to a default font configuration, which is probably Sans 9pt.
In Qt6 apps it continues to be applied normally.

EXPECTED RESULT
Font settings, including font size, continues to be applied to Qt5 apps.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (6.6.37-1-lts)
KDE Plasma Version: 6.1.2
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.2
(Qt5 Version: 5.15.14+kde+r141)

ADDITIONAL INFORMATION
For every app where this bug affects, this message appears:

    QFont::fromString: Invalid description 'Noto Sans,11,-1,5,400,0,0,0,0,0,0,0,0,0,0,1'
    QFont::fromString: Invalid description 'Noto Sans,11,-1,5,400,0,0,0,0,0,0,0,0,0,0,1,regular'
    QFont::fromString: Invalid description 'Noto Sans,11,-1,5,400,0,0,0,0,0,0,0,0,0,0,1,regular'

(Substitude "Noto Sans" and "11" for the general font family and size chosen in the Fonts KCM)