When a mob or player in multiplayer looks left/right (Yaw axis), the rotation of its head stutters because it's not interpolated on that axis but when they look up/down (Pitch axis), the rotation is interpolated.

When in thirdperson, your character's client-side head interpolates on the Yaw axis but doesn't on the Pitch axis. The bug can be fixed by making the player's client-side head not interpolate on the Yaw axis.

A code analysis would be great to correct me if I'm wrong.