For future reference.
Problem: When setting a different wallpaper for each monitor in a dual-display setup, Windows 11 will arbitrarily change both wallpapers to another random wallpaper (in my case it often chose one I was using last winter) or it will change the wallpapers to a solid colour (the same for both monitors).
Solution: There is no solution! At least not one that I found that worked reliably. The fix is to simply stop using a different wallpaper for each monitor. Once I went with the same wallpaper for both displays, the problem went away. I assume this is a bug/quirk in Windows 11 and will be one of those things some people may never encounter and others, like me, will never have fixed.
Alternate Solution: Buy a Mac. This works without issue on Macs, you just have to live with the other multi-monitor weirdness, like “Why can’t the dock stay on both displays at the same time?”
