Same thing happens for me fighting world bosses. If I can keep them off their spawn in point I do fine, but if they disengage the fight and run back to their spawn in they practically instantly heal back to full HP.

May try with a thrall next time, maybe the thrall will keep it off it’s spawn in while I heal up. That is usually how I fight world bosses, but thralls are even squishier now. That is why I didn’t try with one.

Usually my thrall is just backup while I roll out to heal.