I have encountered this bug regularly since the update to Plasma 6. When the screen is about to lock after a certain time of no activity, I sometimes move my mouse to keep it from locking, but rather than stopping the screen lock from happening it just goes to a black screen with cursor instead. I noticed that if I press the Power button of my laptop a few times, it snaps out of it and I'm back in my desktop session, without having to enter a password. So it seems to be a bug in that feature where you can prevent a screen lock by moving your mouse or pressing a key in the first few seconds of it locking. Plasma 6.0.4, Qt 6.7.0, kernel 6.8.9, OpenSUSE, Wayland, AMD Ryzen with Radeon Graphics
*** This bug has been marked as a duplicate of bug 476567 ***