I tried to change one of the backgrounds I use in World one, and the problems started after that. The new one got stuck in the Atlas, even though I don't wish to use it anymore. When I want to delete it inside the Atlasses, it says that it is used by an Object. So I went on and deleted every single object to find which one. None. Not even the tool menu deleted it. Not Optimization or rebuild. In fact, now I deleted all the worlds and UIs, and it is still there, in the Atlas, whit some other unused backgrounds. Did you experience something alike? How can I fix it?