The Bug:

The vertical rotation of players' heads is fully synced with the movement of the camera, unlike the horizontal rotation.

When in third person and moving the camera left or right, you will notice that your player head moves smoothly with a slight delay and this is intentional per the triaging of MC-258579. However, if you were to move your camera up or down, you'll notice that the movement of your camera is fully synced with the vertical rotation of your head, therefore introducing an inconsistency.

Steps to Reproduce:

1. Enter third person and move your camera left and right (horizontally) constantly and watch your head closely.
2. Observe how your head rotates smoothly with a slight delay.
3. Move your camera up and down (vertically) constantly and watch your head closely.
4. Observe how your head is fully synced with the movement of your camera and doesn't rotate smoothly with a slight delay.

Observed Behavior:

The vertical rotation of players' heads is fully synced with the movement of the camera whereas the horizontal rotation of players' heads isn't fully synced with the movement of the camera causing it to move smoothly with a slight delay.

Expected Behavior:

The vertical rotation of players' heads would move smoothly with a slight delay, just like with the horizontal rotation.