Trialmaster keep getting back full hp

its happened in a few different maps now since last night, randomly, im assuming its when i get to 75% for the first time freeze part, he will instead teleport and go back to full hp and become undamagable and then teleport again and become attackable but then teleport again and go to full hp and become undamagable. he will do this over and over again. is this a known bug?