From your description, it could be any of several recent issues listed under Known Issues and Solutions.
You might look at some of these headings and their accompanying descriptions in the Update announcements to see if any apply to you:
- Plasma 6.2 might create a black screen when using X11 and default theme (Note that the workaround for this one is to switch to using Wayland, which it seems you’ve already done).
- KDE Plasma hangs on Shutdown, Restart and Logout
- System freeze on suspend
You could also try creating a new User account (for testing) to see if you experience the same; this could help determine whether or not the issue is limited to your current User account.