Got to the Messmer and the Abyssmal Serpent boss fight in Shadow of the Erdtree recently; it's been one I've run at a couple of times, set aside to level up a bit, then came back to a few times. Last night I got a couple summons (I moved recently and finally have reliable internet!) to join, we got to phase 2 of the fight, and Messmer jumped behind the altar and just kept jumping around not attacking us. I'm a slob and don't have a preferred bow equipped, but one of my cooperators started spamming arrows. Messmer's health bar hit the bottom, then the cooperator turned to me, bowed, and returned to their realm.
I can't tell if they whittled the boss down to its last speck of health and then left me holding the bag, or if this was a total glitch and there was no way to complete the fight. I tried equipping a bow and firing on them to no avail, so perhaps it was the latter. Annoying, though; it was late and that was my last attempt for the night.
I'm kinda excited to get back to my NG++ sorcerer; they're probably a bit too OP for me to experience the DLC for the first time, but I found sorcery a lot more fun to play than my current STR build. Which is odd for me; I typically prefer sword-and-board to magic in other soulsbourne games.