The bug will only occur on the FIRST time you freelook.
The crosshair deviation is relative to how far you freelook on the first occasion.
Whilst it only deviates by a small amount in the video, it can deviate further though I was unable to reproduce. I've had it deviate so far that it was hidden underneath my radar (placed near the top of the screen).
It makes shooting with such vehicles incredibly when it deviates far as zooming in will make you go off target.
The crosshair will ONLY deviate vertically; not horizontally.