I have been trying to set a new wallpaper since the past 5 days. Everyday I set it in the morning when I sit on my PC, and the next day when I open it, after shutting it down the previous night, the wallpaper would have changed to the default one.
This had happened earlier too, when I had been trying to set the former wallpaper around 3 months ago. But I cannot recall how this issue was resolved.
Right now if I drag an image onto desktop and click on “Set as Image”, nothing happens, and when I add the image into the wallpapers tab in Settings, it works, until it changes back to default the next day.
If your Windows OS partition isn’t automatically mounted it won’t be able to access the files and will revert back to the default. Is that what is happening for you?