The PTR patch notes say: “AI will more highly value preventing a mercenary camp from leashing.”
I did Blackheart’s Bay with AI teammates, and Lucio decided to attack a siege giant camp. He went up, stood in the smoke, and began shooting one giant. Since he was hidden in the smoke, it kept regenerating faster than he did damage.
Later Tracer came to help him. She stood out of the smoke closer to the giants, but did not attack or do any other damage. She just waited.