Descending the stairs from the world map to PC: 1, nothing happened. However, on returning with Kenny in tow, as I entered the room containing the up stairs back out to the normal world, a Mirror Room ("the walls have mirrors with many distortions of your image...") was triggered.

I've got the save available for you to check, but as the room's already been triggered, not sure if it'll be any help! I couldn't find it in a search, whether there was a deliberate change made or not.

EDIT: Oddly enough, on hopping into the Puppy Cave, I had a 'many books' room and a 'glob of mud' room *both* trigger no problem on entering the room. So maybe the 'mirror' room is the only one that doesn't work this way? Or something odd happened *only* this once? I... don't know. o.O