I think it is the problem of mouse look which was implemented long after all aircraft models were done.
Mouse look not only rotates the pilot's head, but also moves it, so when you look left or right the head also moves to the left or right. It looks natural and makes it possible to lean over the side of the cockpit to look down.
However when you look behind the head position also shifts towards the tail of the airplane, this is not very useful, but just a side effect. I guess on some models it can get too close or even inside the pilot's seat or fuselage.
That's why you may see "holes".
But it is just a guess, can't test it right now.
Maybe the default head position should be moved forward, or rather the mouse look should be updated so it doesn't move the head backward too much.