WEIRD, there is another part of that strange shadow, another rectangular shape, slightly smaller/more upwards on the screen. It sometimes cuts in from the left into the “normal” part of the screen right of the border visible in the above image. This depends on the direction I am facing on the map!
GZ has no FOV slider, and I have not messed with FOV in any ini file so far. Might still be the ultra wide display though. GZ might read that from the OS whether it be used or not.
I did not mess with it, was on 80. Turned it down to 50, the shadow had a different form. Was something small and irregular in the bottom center over my gun.