As I have mentioned in the description, this is a very elusive and random bug which occurs under the multiplayer conditions whereby a group of players cannot ALL board an MH-9 aircraft if a player(s) has died or been disconnected while still inside the helo. This means the MH-9 cannot be fully occupied by players during transport.
I have seen this bug myself in both Invade & Annex, Wasteland and King of the Hill gamemodes, which are separately made mission files- which is why I am confident this is an engine bug, somehow caused by the addition of the second action menu option for the Helicopter DLC SFV feature.