Didn't see the naga priestress yet. So, didn't unlock nun door in the basement yet.
Game said I found the key to unlock Princess' door. How? When? Dunno.
Saved Princess. Yeah!
Find out there is a slime girl in the well 2h later.
Proceed to do the basement part.
See Naga priestress.
Find key.
Can't use key.
Get softlock at level 27 and stage 4.
Yeaaaaaah.
Because, I ate 28 girls (10 times the slime girl, *rolleyes*) Bunbun is stuck at level 4, and keeps the "belly full" art when I talk to bunny girl in my house. Can't unbirth, can't dildo. Yet I read it has been fixed. Guess I found a plothole.