I think it's a glitch you see on other bombers as well, when you jump to a turret, it will "snap" to the nearest limit based on your last gunner view. Until you do that, the gun follows your view when aiming other guns, it just can't fire. Same for waist gunners on B17s and such that I've seen. If you're in a tail gun and jump to the cockpit, you haven't "reset" the forward gun's last known angle. It corrects itself if you jump to it.