The FOV lock (double-tab numpad minus/plus) gets reset to default FOV whenever you enter weapon sight (ironsight, optics). Also while in weapons sight FOV-lock is disabled. When returning to normal sight FOV-lock works fine.
It is inconsistent with the ARMA2 implementation which was very useful.
This is especially annoying when you try CQB in first person, because I am missing a third hand continuously pressing numpad minus while navigating through the buildings.