Zombies (and baby Zombies) don't hit all the time

What I expected to happen was...:
I expected the zombie to attack me

What actually happened was...:
It stood there flailing it's arms (youtube link: https://youtu.be/KSgKOrKMMPk )

Steps to Reproduce:
1. Spawn 1 zombie
2. Let it attack you
3. 50/50 chance it flails its arms