Sounds like that issue no 160864 (using EndeavourOS with KDE Plasma 6.x). Sorry, cannot include link in this post, you have to search:
Bug 160864 Screen stops refreshing after save, does not make a restore auto-save for files (kf6)
There is also strace output available.
It seems KDE Frameworks v6 vs v5 issue. Can anyone confirm it? Rest of system works fine, just LibreOffice Writer is freezing while using kf6 and saving a new document. But with kf5 it does not freeze (tested with AppImage version of LibreOffice).
Have to admit, that also some apps, e.g. Thunderbird has issues to handle dark theme properly with kf6.
Current workaround seems to change run command: SAL_USE_VCLPLUGIN=gen libreoffice
I’m glad you posted this, as the original poster of 160864, the libreoffice bug did start with an upgrade to plasma6, but I didn’t see how that might be related really, though obviously is. I’ve not run into other apps with similar issue, and don’t currently use thunderbird as you indicated.
Testing this morning using the “SAL_USE_VCLPLUGIN=gen libreoffice” does seem to resolve my issue for now (though makes libreoffice an eyesore), at least a reasonable workaround until their fix gets out to packaging for the kf6 problem.
I have to admit, that with x11 is LibreOffice extremely fast. With KDE Frameworks 6 is much slower. Difference is in multiple times. Depends how KDE can tune the framework but it might happen, that I remain for a while onto x11 until KF6 is working fast and normally again.