U2415, flickering/horizontal scanlines effect on unplugged display

I haven't used this external monitor (Dell U2415) for a long time. When I plugged it in recently, everything worked fine for the first 5 minutes.

But afterward, horizontal CRT-esque scanlines started appearing and going away every ~5 seconds. Half the time, 1/3 of the screen is obstructed by black pixels.

The issue persists even when the monitor isn't connected to anything.

After doing a hard reset, factory reset, testing with another laptop, updating drivers, searching for solutions on the internet, I wasn't successful in fixing it.

Feedback is greatly appreciated!