So this mission, 3 of us running it. The first named bokon first charms the shaman, then the tank then the bard. The mercs are able to kill him, but none of us got the update because we were all charmed. We've had to drop it twice so far because it won't give us the update.
Yes this is an issue if the task requester is charmed when the first bokon named gets killed, task won't update. Needs fixing otherwise as you said drop task is the only option (and redoing it burning the bokon before he charms).