I've even turned up the AI to full skill levels with no change.
What's strange is that I have one mission I created where I fly over some AI tanks before I get to my actual target. After those tanks and my gunner spot each other, then I fly 3km to the next waypoint where my gunner spots the lone AA soldier from up to 1.5km away.
So, the first spotting of enemies by the gunner seems to need them to be in his face, while the next spotting is better even though it's in an entirely unrelated area.