Wallpaper reverting to default - Sonoma 15.1.1

After a clean install (General > transfer and reset > Erase all content and settings) of Sonoma on a Macbook Air M1 I am having problems with the machine seemingly randomly forgetting my wallpaper, which was set by adding my own photo to the wallpaper photos. The machine will instead revert to a picture of a bunch of trees. Curiously, after a restart, it comes up with my wallpaper on the login screen, but once logged in, the default trees reappear. I have to then fiddle with the wallpaper settings (see next par) or log out and log in again to get my wallpaper up. But it will then randomly disappear again when switching between windows or other operations – I'm not sure if this only happens when using Stage Manager because the problem is difficult to reproduce (apart from when restarting the machine) and I haven't taken notes of what I was doing each time it occurred, but I will.

When trying to reset the wallpaper in System Settings, it shows my wallpaper selected (at the top of the settings window). But the wallpaper itself will not show in the background. If I click on "fill to screen", "fit to screen" or "stretch to fit screen" then the correct wallpaper will reappear in the background.

Note everything was OK before the reset when I was running the same OS.

Any ideas? This is really annoying.

FURTHER NOTE: "Show in all spaces" is selected. The trees that keep showing up as wallpaper are selected in "Screen Saver", but the machine isn't in screen saver mode when they appear.