Difference in titlebar between internal en external monitor

Hi,
I use KDE Plasma 6.6.4 on Fedora Kinoite. I use the PurPurNight global scheme and the window decoration as well.
When I put a window in front of a bright part on the rear window I see 2 different things:
external monitor: the title bar has the right chosen from left to right:

Screenshot20260513153703
internal monitor: the right ±20% of the title bar is transparent

Screenshot20260513153911
The white part on both pictures is a part of the rear window.

Any idea how this is possible?

does the same thing happen when using the breeze global theme?

if not, it would be an issue with the theme you are using.

Thanks, that was it.