Happens on various things, Half Tracks, HMGs, LMGs, Brownings, MG42, T24 premium MG, etc.
Its random and all over the place.
Bug report Community Bug Reporting System got ignored for ages.
This needs serious sorting, prevents aiming on T24, MG13, ETC.
I think it has to do with graphics settings because I lowered my stuff a little and I see them less often unless they fixed those certain ones or try increasing your grpahic settings if your setup can handle it. I recent had it happen on the stinger and it made it appear with its old iron sights