When doing an Archon Boreal hunt, I discovered a bug that made Archon Boreal unable to be removed from his invincibility state in the knockback sphere. I think it was because of my Revenant's Mesmer Skin playing havoc with Boreal's bubble. Not sure but when I stayed out of Boreal's bubble he was able to be taken out of invincibility by my teammates, and when I went back in he wasn't.