It always defaults back to one of the new animated ones, and I always manually set it back to the solid black color option.
Because of how the new system work, I wouldn't be surprised the bug is limited to those solid color backgrounds (which are handled quite differently for historic reasons, and it could very well be why you get the error).
The new wallpaper system is a bit of a mess, and solid colors have always been treated weirdly in their preferences.