I think we are in violent agreement. 🙂 I'm also saying it is broken for the exact reason you described. The below radar bug is present for all aircraft. Not just Nightbird. We notice it more with Nightbird because its easier to exploit with that one due to its speed and maneuverability allowing it to dip below radar after the locked missle is in the air.
There is a bug report for this. But the submitter doesn't realize why it's happening. They even attached a video to the bug report. You can see the aircraft is hovering just above the 30m mark and thus the FXM is able to lock and fire. Then, just after he fires, the heli dips below 30m and the missle flys off. This happens twice in the video, back-to-back. and it's a transport heli in this case.
https://answers.ea.com/t5/Bug-Reports/FXM-33-AA-Missile-misses-the-target/m-p/12353354#M25343
Made a video on it here: https://www.reddit.com/r/battlefield2042/comments/11m1nh6/below_radar_debacle/
