All used vehicles, including helicoptors, vanish after being used and then left for more than ten minutes.
This is infuriating and had actually been fixed in the 1.0 release, so somehow this has been reintroduced.

This ruins two key aspects of gameply.

1. There is no point spending time and resources repairing a damaged vehicle, if it's just going to vanish anyway, might as well just spawn another one.
2. There is no point driving supplies up to the front line prior to assaulting a base if by the time you complete the mission the trucks and supplies have all vanished.