Mephisto Regeneration (not using souls)

Why can mephisto regen like this way?
+26 +38
Not at the beginning, about half of the fight. It regen like this.
At the beginning

At the half of the fight



No aura on him

+26 +38
Not at the beginning, about half of the fight. It regen like this.
At the beginning

At the half of the fight



No aura on him

0
Comments
And cold snap might be active on him , due to some buff icon glitch it not been shown
But he didn't have detrimental effect on him.
So it might be a bug?