Possible the fight bugged out like how Will of the Moon often did when that MSQ first came out, where the game actively prevents the enemy from dying before it executes a certain mechanic. Problem with that fight was that sometimes he would just never do that mechanic, so you were left with a boss that couldn't be killed and all you could do was let yourself die or let the timer run out. I had that happen to me three times in a row and wasn't able to beat it until the fourth go.

Didn't have any such issue with this one though, but if he survived several gcds at "0.1% hp" and an empty hp bar without actually dying, chances are something like this happened to you.