Game Version number: 1.3.0.130 (steam)
Modded?: No but have seen the issue on modded

In ARMA 3 when you turned your head, your sights kept alignment no matter what. In Reforger, sights will misalign if your head is not perfectly in the center of the view. This makes it really hard to aim down non collimator because they will either be misaligned in the case of irons or completely black in the case of scopes. The only workaround I have found currently is to have a key bind to pause head tracking, which is more awkward than ARMA 3's implementation.

Here are videos of arma 3's implementation:
and here is arma reforger's: