Skeletons and strays aren't being healed by instant damage

It's a pretty niche bug but when you use instant_damage on a skeleton or stray that was spawned using the summon command it doesn't heal them. Me and a friend are creating custom bosses with command blocks and realized that they died really fast. We did some tests and it still heals skeletons that spawn naturally and from spawn eggs. Like i said it's a pretty random bug but it does mess up a lot of the stuff we worked on