Dying to a mob makes them invulnerable when fought again

I expected to go back to fight and actually be able to kill the mob (like the Wither Boss, Skeletons, Zombies, etc.).
I think we should be able to kill the mob even if we fell to our demise before.

What actually happened was I was fighting and then I would die to a hostile mob. When I'd return to fight again I could not hit it as if it were not there or as if it was invulnerable but it continued to kill me.

I was going to attach a video but it was to big so I'm putting it up on YouTube labeled "Invul Bug Sample" if you wish to see what I was talking about. It will be up by tomorrow march 19.

Steps to Reproduce:
1. Fight a hostile mob
2. If you are defeated, return to fight the mob again
3. Realize it is invulnerable (probably 80% of the time, some times fixes self but not always)