During the personal story “The Source of Orr”, the Sovereign Eye of Zhaitan will sometimes will fail to transition to a new phase of the fight. He will just sit in the middle of his pool perma-invulnerable while he and Trahearne go pew pew at eachother. I’ve had it happen to me multiple times. I don’t think it matters what percentage of health the eye reaches, because it has happened to me when the eye was above 80% and below 20%.