I have a Mac Mini 2018 with 3 identical Asus 32-inch monitors currently running but this same issue has been around for a while, on the iMac 5K that the Mini replaced and the older iMac that preceded the 5K along with several versions of MacOS.
When I restart the computer or start it up after being off for the night one of the monitors will revert to an Apple design wallpaper from the photo that I had set. When I go in to change the wallpaper back to that folder it is there and I just have to click on it to get to it. The other two screens retain the pictures they are set to.
I have isolated it to the display showing the Menu Bar on the screen arrangement window. If I change it to a different monitor by dragging the menu bar in the Arrangement window and then restart the issue follows to that monitor. The other monitor(s) retain the photo(s) set previously. All photos are standard JPG's loose in the Photos folder.
Deleting the plist doesn't help, the problem reoccurs, as it does after zapping the P-Ram in older OS's.
When I restart the computer or start it up after being off for the night one of the monitors will revert to an Apple design wallpaper from the photo that I had set. When I go in to change the wallpaper back to that folder it is there and I just have to click on it to get to it. The other two screens retain the pictures they are set to.
I have isolated it to the display showing the Menu Bar on the screen arrangement window. If I change it to a different monitor by dragging the menu bar in the Arrangement window and then restart the issue follows to that monitor. The other monitor(s) retain the photo(s) set previously. All photos are standard JPG's loose in the Photos folder.
Deleting the plist doesn't help, the problem reoccurs, as it does after zapping the P-Ram in older OS's.