On a fresh boot, opening Settings resets my wallpaper to the default nebula background.
I opened settings again and pulled wallpaper up again after correcting it, and it seemed to apply the current background..
It seems whatever Settings 'remembers' gets applied as soon as one selects the Wallpaper section, so on boot, all there is is the default so it goes with that?
I think simply not automatically applying wallpaper unless explicitly told to do so would be the solution here.