I tried a few times to do the mission this reset, and found that the bug making the Celebrant immune to damage occurred. I hadn't had this problem before. So, I tried slowing down as asked- I was pecking a few kinetic AR bullets at it at a time, taking at least five minutes, but when it finally teleported across the first room, he became immune to damage again. I don't think this is how it was working before, and may be a new development, as I was definitely not going too fast.