Seems to be something to do with Compositions potentially? Kept running into issues with all vehicle doors being obstructed (although they could be opened) and preventing access/exiting when near stuff like supply containers as well

I've seen this as well while testing in the workshop

I'll add this:

A large proportion of the time when I land on a helipad I cannot get out of the helicopter, not matter which seat I jump into. I have also experienced this with the helipad setup on the airport. To get around it and attempt again to get out, I have to take off and fly back around. Neither the action on the door, or the bail out (2xX) lets me get out until it has seemingly reset the logic.

Tried this in game master mode and was also able to replicate it so its not mission specific. If you place a helipad down and try to land on it, this is seemingly what triggers it. The same thing happens when you land at the FARP's in Air support, so its potentially the object or how the helicopter interacts with it.

However, if I land on grass the other side of the helipad, I can always get out.

Eno suggested the following might be the error on discord:

I think something with the vertical transformations were messed up for vehicles and treats it as if the vehicle is stuck in the ground and obstructing enterences

So, to cap off, here is you reproduce it:

1) Acquire helicopter.

2) Fly to helipad object (place it down, or go to a FARP)

3) Try to get out, using either F key, or Bail Key. If it works, go around again and try again as it happens about 75% of the time.

4) try to land on grass, and you will always be able to get out.

This continues in 1.9.0.10. It is impossible to board a helicopter that is parked on the helipad. By the way, it is impossible to board any vehicle (including a car) on the helipad.