Seems that the problem still persists with this mission.
Not only are pets dissapearing now but I had an odd occurence last night while in a team with the 2 warrior henchies.
After killing the first dragon we entered the portal and all arrived at the new area, only to have the two warrior henchies run off at great speed across rocks and non accessable areas. Fastest I've ever seen those two move.
They came back two dragons later and stayed with us for the rest of the mission