Hi - I do wonder if there is some underlying shared cause between what you’re seeing, and what’s described in that bug report, which might be triggering the “right” conditions even without the monitor going to sleep.
Does adjusting the size of the Plasma panel back and forth by a pixel correct the issue, even if temporarily?
I’ve checked man notify-send and skimmed through the 9 pages of Desktop Notifications Specification but did not find anything related to x/y position nor window placement.
Hmm - a couple general troubleshooting steps might be helpful to at least rule things out. Could you try:
Creating a new user account, and seeing if the issue shows up in that account? (To rule out an issue with the specific configuration files and settings for that user)
Checking the system journal (logs) after the issue occurs to see if any errors related to the display, shell, notifications, etc. specifically show up when it happens? Running sudo journalctl --since=-1m would give you all of the journal entries in the past minute, which may hold some clues?
Logging in to a Wayland session to see if the issue occurs there?