I encountered a monster zoo (I got the achievement for discovering it). I then killed a few enemies, hopped on a pentagram teleporter, worked my way around the map, and re-entered the monster zoo from the top (different entrance). I killed every monster in the room, but the counter remained the same for the remainder of the game, and I didn't get the achievement for beating an entire monster zoo.
Yea if you walk too far away it makes it so you dont get the item/achievement, also breaks if you go to a different floor. This is intended so you have to fight a zoo right away and without going around to clear the map/heal first.
