Mobs can tilt their head 180° when ridden

The bug

Recently I've been testing some vanilla Custom NPCs with the "{Passengers:[

]}" tag, and I couldn't help but notice when the lower mobs head was able to turn 180°. This happens with mainly humanoid mobs. It also only happens when the mob has a Passenger Via Passenger tag. I've provided some images that show what this looks like.

How to reproduce

/summon minecraft:zombie ~ ~ ~ {Passengers:[{id:zombie}]}