Hi, I'm John!
I recommend submitting feedback through the Feedback Hub first, especially since you've already tried turning off sync and clearing the CachedFiles folder without success. This helps Microsoft investigate whether this is a Windows 11 bug.
To submit feedback:
- Press Windows key + F
- Select "Report a problem"
- Describe the wallpaper reverting issue and submit the report
In the meantime, try setting your preferred wallpaper again, then restart the PC and check if it remains in place.
Also, could you provide me with a screenshot of your current Windows OS Build?
Press the Windows key + R to open the Run dialog, type winver and press Enter, capture a screenshot (make sure not to include your email address)