I have experienced this before, just not mid questing without logging off in between. Yesterday when me, Schoan and Valdaynie were running Harbinger of Madness chain, the hireling suddenly bugged. When we exited Sinister Storage, instead of the hirelings leaving the party on exit as they’re supposed to, they stayed in party. The hirelings were not actually there, we could not see them, control them or kick them, but they took up their assigned party slots, and as we entered Fear Factory we could not re-summon them as the game told us they were already there. Check out the recording below.