Stone Citadel boss does not phase transition consistently

Relatively regularly when doing the stone citadel boss, he does not immeadiatly phase transition when he hits the health threshold. In this state he will continuesly attack, with you having no means of retaliating. You are essentially forced to just wait and hope you live long enough for him to phase transition.

I am not sure what triggers this bug, but it happens commonly for me. It seems to have increased in frequency the more dmg my character has started doing. But that might just be annecdotal evidence.