Reloaded old save, tested, same thing happened. I won't restart the game from start for this btw.

Even if it didn't fired upon us, i looted a RPG-42 just to bring it down with AT rockets.
This mission should be done like this, it's more exciting to kill an attack helicopter which prevents you to pass.

It happened to me once. (both mates died)

I restarted the mission, and managed to save one guy, not the team leader.
Be sure all enemies are killed too.

No problem after that