Zombies and skeletons won't die

I made a world where I gave myself a bunch of effects at level 255 mainly resistance and strength. I was playing in this world grinding mobs when I saw a skeleton with a full set of gold armor, so I attacked it but it didn't die, so killed it with the kill command. I thought it was strange so I decided to test it out I spawned a bunch of zombies. I easily killed all the zombies without armor. All the ones with didn't die. I made a zombie villager trap and lead a zombie with armor in and placed lava above it, it died. I thought it was the strength effect I gave myself, so I opened a new world and drank a normal and level 2 strength potion, and easily enough the zombies died. I then gave myself strength level 255 and spawned in a bunch of zombies. I tried killing the armored zombies, some died some didn't.

Please fix this so I can kill armored zombies and skeletons with a high level of strength