Sorry to recap, but thats not "fine". Im sitting the Drivers seat, i use the Context Menu to turn of the Engine, i use the Context MEnu to get out of the Vehicle. And exactly the moment i step out of it, the Engine starts aggain.

Why could that be intended? To make even more noise and have an running Vehicle wich consumes Gasoline whilst noone uses it? Every other Vehicle stays off until you hop in it and get going. Just the Quad decided right after last patch to start the engine even when noone is in it anymore and the last Driver decided to turn it off.