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I can't really help since i'm not a dev, but it may be because your level was too high for the boss (even if it's unlikely considering you were playing on Nightmare).
Oh and, did you use several Burst for his last phase? Normally that's how people beat it without getting hit more than necessary, though you can still beat him without.
All I can say for sure is that, at level 50, you definitely should've been taking a fair amount of damage from his attacks.
Maybe that's the same thing that happened to you.
If the invincibility persisted beyond a couple seconds, then yeah that's a bug. I suspect it just missed the timer to turn it off or something.