That is strange, because the "Full Wayland" session for both is identical (or at least supposed to be). The only difference is that "differentxwl" still offers the "XWayland" session, but by patching Qt (which isn't great).
Apparently it had to do with how the enviroment variables were ordered: if GDK_BACKEND was set to "x11,wayland" it preferred x11, but if it was "wayland ,x11" it went for wayland.
Do you have a link to the upstream reports or have a backtrace?
No, sorry. I could try to troubleshoot a little bit but lack time right now.
Is this using the compose key or just plain dead keys as part of the keyboard layout?
Compose key - this was fixed recently: https://bugs.kde.org/show_bug.cgi?id=411729 https://bugreports.qt.io/browse/QTBUG-87088
What made the biggest difference?
For me, the fact that Vivaldi, VS Codium and Element were working without touching enviroment variables.
change the default in qtbase to "wayland;xcb" and drop the "XWayland" session
In the case of GDK_BACKEND, what would happen?