Like the title says this bug istantly heals A boss heatlh meter to its max.
Can you record and link to a video of this bug occurring?
I believe some bosses are designed to gain health back over time, but I could be wrong. Like Chimera said a video would be helpful.
Some bosses do regenerate health (mantis) and watchers can heal them as well.
Watchers can heal any Promethean boss instantly which is why you have to kill them quickly. And I think Tankmaster Rok regains health slowly if you don’t keep putting shots on him.