When opening the door to a zoo the game seems to go into "zoo mode": The music becomes rather climactic and you get a message "n monsters in zoo left". Now when things get heated and you move to a different floor to heal and then back to the active zoo floor this will break the zoo mode: # of monster message is gone and the music will cycle each time you kill a zoo monster. Also, after I slaughtered this zoo population and entered the empty zoo room, there was a rather lonely Eely boss sitting there stuck in the floor. Not really a bug I guess but it looked out of place.
I had this problem too. Found a zoo and immediately slammed the door, then went on exploring. I ended up going to a different floor before I came back, and when I did, I didn't get the achievement.
