I can only type letters with accents such as à or é in applications that use xwayland. On others I can't. Also sometimes I get very funky glitches where typing just the accent (in the Spanish layout you type the accent and then the letter so `+a=à) triggers weird global actions: touchpad disabling or opening the activities bar. I have some debug information in place but it hasn't been all that useful so far and it's not trivial to debug because it's not easy to reproduce, especially the global actions issue (which could very well be worthy of a separate bug report).
Did the debug show if the accent key was always being eaten by the GlobalAccel event filter?
I have a theory. It's a long standing bug and this is why Martin changed his surname!
#Comment 1: when I get the weird touchpad thing, I do see kglobalaccel returning true. In any case I can't type accents on wayland applications, hence my proposal to fix the accent thing first and then we look into kglobalaccel. #Comment 2: x'D
It is a long standing bug in Qt: it doesn't support compose key (see https://bugreports.qt.io/browse/QTBUG-54792 ). If one starts the Qt applications with xcb platform, you will observe that it magically works. Unfortunately my new last name still has an Umlaut.