Inconsistent mob melee attack rate / mobs double-attacking

The bug

Enemies like Zombies and Endermen are sometimes suddenly attacking quickly 2 times in a row instead of at their normal attack-rate. (Melee attacks only as far as I know). Sometimes the attack animation or sound won't even play.

This can seemingly mess with blocking too, though it could also just be MC-100949

Code analysis

Code analysis by Chumbanotz can be found in this comment.