If you are right behind the guard his body should actually block his vision. If that's not the case then I would consider it a bug. But you really would have to stand behind him, not somewhere to the left or right of his back. If I find the time this weekend I'll take a look.
Regarding rng: If the player is aware of the head turning, then all it does is to increase the ai's fov, with the difference that there is an area in front of the ai where you are always visible and the area to the sides where you are only occasionally visible. And as the ai doesn't tend to turn their head like a lighthouse tower there should be an area in their back where you cannot be seen.
From my experience this is the case, but as stated above, there might be occassions where something goes wrong. However, I wouldn't consider such a setup random. If you stand at the ai's side you know you will get spotted. The only thing you don't know exactly is how long it takes.