Dunno if this is a bug or just weirdness, but in Vince's Vault there was a subdungeon with one monster completely walled off that I could not seem to access at all (I don't recall finding any glyphs that would have been helpful).
screenshots:
the subdungeonthe dungeon(As an aside, I think it's a little weird when the dungeon edges end in blackness, rather than with walls, at least when the dungeon is not taking up the full screen.)