I've now experienced this on two toons, a lvl 42 fire/dark assault dom on Excelsior on map Office_Layout_04_01 and a lvl 45 fire/elec corr on Excelsior on map Office_60_Layout_03_02. In both cases at the end of the map summoned pets and mob (when attacked) teleport out of the room to somewhere back on the floor. The first time I forgot to note the mish, this time the mission is "Rescue Howard Quigley" from Janet Kellum. It doesn't make the mission impossible to complete, but seems like it could be a problem if mobs are teleported outside the map.
Tim