RathBreaker doesn't react

Bug Report ID: #422064023

I just got to rathbraker and if you kill his 2 lieutentants from below the cliff, he doesn't seem to turn on aggro, he did one attack and then I fled and he went back to his initial position and had more regen than I could do damage to him (normal playthrough). Using a huntress. I left via checkpoint then townportal. Happened again.

then I respawned at checkpoint via game menu. Then he activated