The issue would happen in other occasions as well but it was fixed. then, after a while, I also found out what you just described (about 8 months ago) but to me it only happened if I tried to enter the mirror world "after" the first time. meaning in each playthrough, entering the underground other world was fine the first time (when you had to) but it gave me the black screen every time after I got the key, and not only THAT specific other world, but every other world in the demo after that. I am sorry to say that we still didn't have any update on the bug ever since but I do hope Orange fixed it in the full version ^^