Fluid dynamics in ballistics 101...
Ok here's the brief run-down of what I think is going on, based on what you wrote:
Any object going through air experiences a multitude of forces upon them, bullets are no exception and in fact due to their high velocity and small mass, the aerodynamics are quite complicated (air tends to become quite viscous at such high velocities, as an example).
Not only do they experience high drag, but the airflow can also exert sideways forces on them.
The biggest issue here is the
Magnus effect. It is essentially the same phenomenon that causes the curve in a curve ball on football, tennis or what have you.
Magnus effect is caused by the rotation of an object traveling through airflow. The rotation causes situation called
circulation, which makes the rotating object behave analogous to an aero foil - lift force is exerted upon the rotating body.
Bullets rotate very fast due to the rifling of the barrels, so if they travel even the least bit sideways through air, they can experience very high sideways lift forces that can curve their paths quite a bit.
When your airplane is traveling forward at low angle of attack, the guns are fixed to point forward as well - meaning the bullets will travel roughly against the airflow. When you are pitching up or down, or yawing left or right, your airplane points in a different direction than where it is going (heading and velocity directions are not the same). This means that your guns now shoot slightly sideways into the airflow, and your bullets will experience Magnus effect, curving their trajectories in unpredictable fashion and increasing their drag.
This effect was in fact so significant as bomber aircraft got faster and faster that shooting to the side, or directly up or down caused the bullets' trajectories to curve so drastically that it was practical impossibility to actually hit targets that way. Fighter pilots (those who lived) quickly noticed this and approached their targets from above, below or the sides, as the gunners were much less effective that way compared to frontal assault or shooting from 6 o'clock position. The bombers countered by flying in larger formations...
Now, not having seen the issue in-game, I can't say if it is in fact the ballistic simulation of Magnus effect that is causing you to miss, but I did hear they supposedly upgraded the ballistics quite a bit. Maybe this is one of the improvements?
Don't take my word for it, it could be a glitch as well as accurate simulation. Wait for a dev to give you an answer directly from the horse's mouth, when they have time for it.
Incidentally, this is one of the reasons so many tanks use smooth-bore barrels on their main guns as opposed to rifled guns. Smooth-bore barrel does not give the benefit of stability to the projectile, but on the other hand it is much less affected by wind for reasons I specified above. Stability of the projectiles can be achieved by other means than rotation from the rifling, after all (SABOT rounds have flechette-like projectiles with fins for stability, for example).