Usually, when you hit a mob, regardless of if it's a crit or not, the mob takes knockback. However, if the mob being hit is on fire, and it is hit with a crit (critted? I'm not sure of the right word), it sometimes* does not take knockback. I commonly encounter this bug in the morning, when the zombies and skeletons are just beginning to burn in the sun.

*After initially encountering this, I did a bit of testing, and I couldn't find any way to consistently get the mob to not take knockback (although the bug does happen most of the time). It seems to me that if all conditions (listed below) are met, the mob may or may not take knockback. It seems to me that it's random, and if it isn't, then I have no idea how to consistently reproduce it. 

How to reproduce:

1. Find or spawn a mob.
2. Light it on fire. 
3. Jump and crit it. (The amount of damage inflicted by the hit doesn't matter, as long as the mob doesn't die.)
4. If the mob takes normal knockback, make sure you've followed the above steps and try again a few more times. 

OneDrive link: https://1drv.ms/v/s!AqPWodVZ2UX0ghp61QTY-fMlHJbQ?e=NJePR4