IDK if it's a bug or intended behaviour. I noticed that HUD in su34 acts differently than any other airplane. If my understanding of collimator optics is correct, the HUD should remain in the center of the screen when your head gets offset sideways. In every airplane except su34 it seems to be behaving that way. However on su34 hud seems to be "glued" to the glass. It makes it really hard to use it for aiming when pulling Gs because pilots head gets moved around. On the attached image is a comparison with SU25. Notice how with SU25 hud remains static relative to the structure in the background (red line)

STR:

1. Spawn a SU34
2. Get into first person mode
3. Offset your head sideways (default binding ctrl+num4/6/pgUp/pgDown
4. Observe the HUD