Takedown completion problem

The past three or four Wotan fights have been getting bugged for me (and my group). Everything is fine until Wotan splits apart. His torso seems to leave the arena and while out adds spawn out of bounds. His torso also seems to just crash to the ground after a couple shots (after I kill his legs)(never thought the phrase kill his legs would be needed). The whole group still gets the loot, but the Takedown Completed message never pops up. I assume this will only really hurt first time runners.