On a client the command will return empty arrays for every unit that is not local. On the server the command correctly returns the role, regardless of the units locality.
When player is in a group of AIs and is not the leader, assignedVehicleRole of the player could be unpredictable and absolutely not related to the position player occupies.
In short, forget about this command for the player, it is unreliable and has been broken too many times in the past.