I noticed this just now, exiting Greenwater and walking towards the ship. During normal walking, when there's the usual head bobbing, the whole sky / background / horizon is shifting and and down, left to right, slightly, with the movement of the bobbing.
It looks super-weird, very distracting and I guess this is not intentional. Probably a bug.
I could definitely try and make a video if you don't understand what I mean, but it should be reproducable. Just walk outside of Greenwater. During the day I guess. And what the skybox.