I actually dont see this as a bug. But just though to inform that this does kinda look silly.
Currently it is possible to have Cupola / gun shield together with weapon station and it looks like this
(hatch actually is closed if you start mission, this is from mission editor)
With gun shield
I tested this and it didn't actually seem to limit view at first. Then I zoomed out and well, then it does.
Again, I don't actually see this as bug, as I doupt anyone sensible would have this kind of set up in the scenario (hopefully atleast) though accidents do happen.
This is just ridiculous set up that is possible to make to some M113s that allow weapon station to be added.