It's actually a fairly widespread bug introduced in morrowind. There are a lot of invisible barriers that mobs (and sorc pets) will not cross that they crossed before morrowind.
PC/NA - Nightfighters, Raid Leader and Officer
Lilith Arujo - DC sorc tank/dps/healer - Dro-m'Athra Destroyer, Gryphon Heart, Grand Warlord
Lilith Tortorici - DC templar trials healer
@Turelus - EU PC Megaserver "Don't count on others for help. In the end each of us is in this alone. The survivors are those who know how to look out for themselves."
I noticed tonight that in BC2 the daedroths didn't come off the platform either. Also after killing Maw the mobs after the door you open me or my group couldn't use any skills for about 5 seconds. Dungeons buggy as hell now.