I have 2 computers running with Plasma and both have this happen. One is has nVidia and the other an AMD graphics card so it does not seem to be dependent on that.
If the monitor is not turned on when the computer is, and left to sit a while (I’ve seen it happen at 45 minutes, but have not tested for a bottom floor), when the monitor is turned on the user is presented with a black screen with mouse pointer. No login background wall paper, no login prompts.
If I access a tty terminal and kill plasmalogin (triggers a full wayland reload looking at the ps tree) it works properly again.
More rarely, I’ve seen this happen if the monitor is turned off/sleeping for a long time while the session is locked.
What might be causing this and how do I fix it?