The radar ping bounces on it's way back from the heli from ground. It's called multipath propagation and that's why the radar is locked to the ground. Happens everytime the aircraft is flying close to the ground.

Thanks for answering! I had no idea what was happening. You can see me looking at the target point from the radar and then noticing the heli was way above it and I managed to score the kill anyways but it's definitely something that has messed me up a few times so far