Yes, this could be the 'Mutiny Clause' I've had this happen with my druid npc/pet user. What I think is happening is: If I leave that npc Or pet someplace not within my vision range "told it to stay' or stand by a lever, Or it got left behind because it didn't figure out a doorway etcetera, or it died and I didn't pick up his bones... Then I leave the quest... That particular pet stays in the group and .."I think it's mad, cause it got left behind"... It isn't around once I am out in the explorer.. but is in the group -> while I am in town too? So, when I go to or get to the next quest I try to resummon it-nope, I try to kick it out-nope, can't even kick it out in the quest, I think it's a mutiny and it took over the group (as I believe it stays in the group, even after you leave). So if this is the scenario you're experiencing, yep it'll be a total bust of group and reform. Leave no npc behind and it might avoid the mutiny clause.