I had a similar problem with my last monitor (with a Windows PC.) It would randomly flicker vertical lines, and those lines would show a “ghost image” of something that was on screen when it started. I think it had to do with its variable refresh rate, as it often seemed to happen after gaming or tabbing from full screen to the desktop. But I could never find a reliable way to replicate the bug.
I could always temporarily fix it by running a “stuck pixel” video in full screen for 10-15 mins, like this: (seizure warning) https://youtu.be/-2eQhhMvi-U
It wasn’t really “stuck pixels” but it seemed to blast the problem away for some reason. Could’ve been coincidence though.
If it’s a variable refresh rate issue between the Mac and monitor, you could also try turning Adaptive Sync off if you don’t need it: https://support.apple.com/en-us/102144 And also disable it in your monitor’s onboard settings if you do turn it off or already have it off.
I went through a few different GPUs over the years with that same buggy monitor, including the GPU I have now. I since upgraded to a new monitor, and this one has not had any problems in the 6+ months of using.
So I’d hazard a guess that it’s the monitor. Try the pixel video if it happens again or messing with Adaptive Sync. If it doesn’t work, try other similar videos. Maybe it will help.
Try tap.
Did it instantly turn on? No?
Try hold.
Now you turned it off, maybe.