Hi, I have a very similar bug where kwin-x11 crashes, I get logged out, but have to manually kill kwin. But that bug appeared just recently (I'm using kwin-x11 5.24.4-1 from unstable).
Usually after logging in I use a hotkey to go through all windows that want attention (after logging in it is all of them) and around half the time I do this kwin crashes. This maybe happens because I switch too fast through the windows? I haven't found any error yet, but this could be related. Best regards Matthias