Bug: Attackers not dying from being shot. Actually gain health

So I came up to Crevasse Dam and ran into super mutants.. start picking them off and I notice they are taking damage. As soon as they got low enough that they were about to die they start gaining their health back. They weren’t legendary. Continued shooting and couldn’t kill any with my guns. Had to go in with the super sledge and that could put them down. Tried again after the exchange with random blood bugs and again guns couldn’t kill them. I can’t find anything online about this bug. Any help?