When taking fire damage as an undead, the player is always knocked back in one specific direction.

I have also noticed this issue but have not identified a reliable way to make it happen. If you could find out what triggers it, it could be very helpful.

Quite intriguing, thank you for the details

Found it. It happens if the mob you're currently controlling has been attacked. The knockback takes the direction and amount of knockback you got from said attack. Easiest way to see this is to aggro a skeleton and have it shoot you, then go get burned, or get a zombie to hit you.