It's passive. He only regens against active ones like bleed/poison/starburst ect
Ghulk doesn't regen from debuffs like bleed or poison effects. He only regens from passive damage over time, like starburst, or dominos critical failure.
To the OP, this sounds like a bug, but I'm not super familiar with the Maw yet. He might have some AAR that I don't know about.
It's passive. He only regens against active ones like bleed/poison/starburst ect
Starburst is passive and he decreases the damage done to him while increasing the attack. Bleed and poison, however, don't count as passive effects and have no effect on gulk's "face me"
Also, would you be able to tell us what quest this fight was in or what node buffs were active during the fight? On some fights, there is a Faltering Mind node that decreases defensive ability accuracy by 100% when Ebony Maw inflicts Falter on his opponent.
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To the OP, this sounds like a bug, but I'm not super familiar with the Maw yet. He might have some AAR that I don't know about.
Bleed and poison, however, don't count as passive effects and have no effect on gulk's "face me"