I have no valid data regarding the reality, but i think if an AA missile is in air and its locked on the Helo with heatseeker, it should not loose track if the heli lands. I mean the engine will not cool down that fast. If it is radar lock, i dont know, possible then. What do you think?
IR missiles shouldnt lose lock unless LOS is obstructed
If it lands radar missiles cant tell it apart from a building and loses lock, as for heatseeker my guess is that the engine cooling coupled with the much lower contrast that the seeker will be seeing makes it hard for it to track, especially if the heli also flares a couple times. But maybe someone knows more in depth why a heatseeker would lose the lock, not a 100% sure on that one
Next time it happens, post a server replay. We can better understand what might be happening.