If I had to guess, this sounds like a target for the mark might be assigned outside of the arena or something like that. Maybe a player died or ported? Or maybe someone was just outside? The boss should maybe have some kind of timer based refresh for the target. Maybe that can solve the issue? Of course that is potentially silly if you are just about to snare him, but probably worth it based on your accounts.
We tried leaving the area once where it just bugged the boss even more. Shades chasing the leaving players and getting stuck at the entrance, where they can't pass through. Having everyone die and not revive until everyone is dead seems to be the solution that works best. If everyone listens...Workaround: All die or runaway, i.e. reset the fight.