As far as I have tested, the issue arises in Invade & Annex and KOTH gamemodes. When a player is a shot out of the MH-9 Hummingbird or disconnects whilst inside the chopper, other players cannot use the context action menu to board the vehicle themselves. (The seat taken up is called 'Error: No Unit' This issue has been apparent since the Helicopter DLC update where the shooting from vehicles feature added a second option to get in MH-9 aircraft as 'Get in left/right bench'.

An example of this issue in action:
-I am flying a freshly spawned Hummingbird to base to pickup troops in MP (client ping: 30ms, server FPS: 40, no server desync)

-I pickup all 7 troops in the helo successfully.

-Along the way, one disconnects whilst we are still flying. This players seat is called Error: No Unit and his body is still visible inside the helo.

-I drop off the 6 troops successfully, then fly back to base again.

-Now only 5 troops can get in the helicopter no matter what I do. 2 on left skids, 2 on the inside and 1 in copilot. No players can board the right skids whatsoever. I tested this with players of varying ping from 20-100ms ranges, with no difference.

It appears the dead/disconnected player completely removes the action menu option to get into a vehicle under certain multiplayer circumstances.

The random occurrences of the issue can be fixed/partially fixed in one of two possible ways:

-Destroy the helicopter/use a new helicopter with no dead/DCed player inside. This will allow a full 7 players to board without error.

-Pick-up 4-5 players (usually this number)that have the action menu option to get in the helicopter. Then, sometimes, as a workaround- the helo can be flown and landed in a different spot and one or two players can board. This fix rarely works and I don't see how it affects action menu availability, but it has worked several times in the past.