If you are using this command as a validation method, it should not be the sole thing you are checking for, as the result is often inaccurate.
For example, it returns false for some helicopters when landed on the roof of certain buildings, and it always returns false for boats, even if they are beached.
Demellion
In addition to previous statement: That behaviour is true, simply because isTouchingGround applied to a player unit (or any unit) is frozen to a last state when unit enters vehicle.
To get proper return from this command you should go for units vehicle:
This command returns always true if the falling object is attached to some other object with the attachTo command, like for vehicle air drop with parachute.