Basically you used to activate a mod and the wallpaper changed to that of the mod (if you had personalized wallpapers and the wallapepr in the mod activated). But now it stays on the one you have as default, and if you change mods it changes to the default too.
Havent changed any settings so it has to do with new versions of GX (at least I think it should be an option to reset wallpaper everytime you change/activate/deactivate mod, or activate immidietly if any mod is activated)
Hopefully my english isnt too bad, and this "bug" (or whatever) is fixed or something idk. Its not like the end of the world if nothing changes