To be clear, there seems to be no difference in h/v mouse sensitivity. The reason why I thought there was is because the game is mostly about turning and not height aim, so I would move my mouse slowly when going up but fast to the sides; and as I mentioned, for some reason moving the mouse slow makes it rotate fast and when moving the mouse fast it rotates slow. And no, I don't have mouse acceleration and all other programs operate normally.
Some other notes:
-This input problem effects the normal aim and free look, it's normal when using the turret barrel zoom aim.
-Opened the game and testing it. I'm fairly certain the problem is that you use a constant speed for the rotation, independent of the mouse movement delta. So if I barely move my mouse(over <inch), I can rotate the camera 180* in the same time it takes to drag across the entire mouse pad. Therefore, the reason I thought it was changing speed is because it doesn't respect my small mouse movements that I am doing for precision. My guess is that you have a normal DPI mouse, and so small movements feel normal since it updates less often or something? Alternatively, it could be that my mouse DPI is so high that it hits the max speed instantly and it's my fault, although other games aren't like this.
-I changed my DPI and the problem persists. Lowering the game's sensitivity makes it more manageable, but the same problem is there.
Hope that helps.