I haven’t played Arma 3 but I fixed a similar problem in Dragon Age Inquisition.
In DAI, the issue came from the fact that camera movement was actually bound to a key. In the normal game, you don’t move the camera by simply moving the mouse, you need to hold right-click while you do it. While using a first person mod with VorpX, not using the mouse caused the camera to simply follow my head movements and stay centered in front of my eyes at all times.
Turns out it was a simple toggle that I just had to deactivate in the settings to fix it.
You should check your settings and key bindings to see if the camera is linked to a similar key.
Hope it works!