Wallpaper resets after some time on Pixel 9a

After applying a theme, the correct wallpaper is shown at first. After a while, the wallpaper switches back to the previous one without any interaction.

This happens at unpredictable times, sometimes after a few minutes, sometimes after several hours.

• happens during normal use, not after a reboot
• only the wallpaper changes back, icons and widgets stay the same
• occurred on Niagara 1.14.10 and still on 1.15.2
• Pixel 9a, Android 16
• no wallpaper rotation or battery restrictions active