Formless One Takes No Damage[DAV ACT 1 SPOILERS]

I've found a lot of references to this issue, but I didn't find any with a working solution I can use.

After flying up to the ledge, Formless One stops taking any damage. The troubleshooting suggestions I've seen don't make any difference, including that I'm doing too much damage too quickly (definitely not the problem) I have to turn off cheats (don't have any activated) or disable mods (I'm on Xbox, so no mods installed.)

After an hour, Formless One had exactly the same health they had when they flew up to the ledge, and never took a single additional point of damage. There are other issues, but my inability to do damage to the boss makes the fight impossible, rendering them moot.

I don't know what I can try to get past this, so this is mostly a hail mary before I give up.