Desktop Background display
I used to have a 2012 Macbook Pro running Mojave. I set it up with 9 different desktops. By right-clicking on a Finder window I could change the background to a folder in my Photos library and choose "change background photos" randomly every minute. This worked fine.
I now have a new 2020 M1 Macbook Pro and am attempting to do the same thing. It seemed to have worked for a while (and my second display continues to work) but now I lose the pictures on the background and it keeps defaulting to the Big Sur desktop picture. Every time I try to set up a desktop background I can do it as long as Change Picture is unclicked. When I click Change picture it defaults back to Big Sur. Sometimes it works, but then at some point later on multiple desktop's backgrounds are back to Big Sur's.
I've tried changing the setting from Automatic to Light or Dark, but eventually, Big Sur returns. I do set them to fit the screen rather than full screen so that people's heads don't get cut off. I've also tried choosing different photo folders within my photo library. No luck.
I tried deleting the desktoppicture.db and restarting. Again, no luck.
Is this a Big Sur or M! issue? Is there a solution to my problem?
Unfortunately, I've tried all of the suggestions above. Only on some of my windows does that work. The rest will default back to Big Sur's default. This did not happen with Mojave and before. I could have all 9 of my spaces with different pictures, each of which randomly changed every minute.
If that is the case we need to isolate the issue. Could you create a new user account, just call it test and give it any password. Login to the test account and attempt to reproduce the issue. If the issue is not reproducible in the test account then the issue would be isolated to your main account, otherwise it is a system wide issue. This knowledge should help us learn what to do next.
To add a new user account see this article: https://support.apple.com/guide/mac-help/set-up-other-users-on-your-mac-mtusr001/mac