That's weird. We thought it was herpaderps at first...but ... 5 pulls later, we had everyone stand on the stairs and just one tank pull the trash and sure enough he came running. We eventually got the trash down...but yeah, he just keeps disappearing at about 50%
We had the same issue last week in 10 man. Did several trys and he always despawned on the mindcontrol platform.
Then wie did a hardreset and did the trash a 2nd time. We think the problem was, that the boss was pulled with the trash the first time. We you pulled the left pack, then the middle pack and last the right trashpack and pull it back to the stairs and the boss didn`t join the fight. And then he didn`t despwan in combat.