Act 4 kaom has a regen issue

Bug refferal 29,852,734

In more of 24 then fights the fire dragon rare monster disipite of his auras and immunity to fire I manage to get his HP down to half but after the fire circle or don't now what triggers he goes to full life no matter the DMG I apply , regen capacity looks like a delve depth 3000 haste regen souleater

My build is a fire build I took this in to consideration but I switch to pure physical and the same result , manage to change instances 3 times