So I think you found a bug, because I tried to recreate your problem, and kinda succeeded.
If you tried to load a map while inside another map, it will overwrite the map that you were in, deleting the new one and saving over it in the process, but wont be loadable.
So pretty much just make sure NOT to load while in another map, only save.
If you want to load a map make sure to go to the main menu after saving.
If you’re still having problems while taking precautions, say what you’re doing step by step so I can try recreating it again. Otherwise hope that helps.