I’ve had this happen twice out of maybe like 40 runs so it’s not a common issue but it sucks when it happens. When he does his multiple plague ranged attacks sometimes they’re going straight down the middle of the arena visually but you get hit regardless of where you on the map.
Just had a run where I was going for the dagger only challenge ended because no dodging or blinking could save me from the damage and it was as I said, visually down the center of the arena and I was not.