Is that a thing? Well, yes. Sort of.
It turns out LCD screen pixels can acquire a retention of images that are static on-screen for a long time. For example, they will show up as ghostly images of desktop icons on parts of the screen.
Often the problem goes away if you leave the screen powered off for a while. Alternatively, you can run the extremely simple Windows program I made. Let it run for a while and the ‘ghosted’ images should go away.
Alternatively, if you worry about malware (and you should), you can download the source code and compile it yourself using the free Lazaraus IDE (https://www.lazarus-ide.org/index.php?page=downloads)
Further testing shows that the above does not permanently resolve the issue. Rather, it must be an underlying defect in the LCD panel of my ASUS ROG PG279Q screen.