This does still happen, once someone hops in the driver's seat of my player-owned vehicle, I can no longer use it as if it's mine.
My suggestion: When player locks the vehicle don't allow anyone to get inside by breaking in or make it take about 15-20 seconds to break into a locked vehicle (Only if player is not near?).
Vehicle steal bug
The bug is most obvious as a cop or EMS because you get a "vehicle requisition successful" (something along those lines) message after a civilian tries to hot wire your personal vehicle. As a cop, you also get a prompt to take your personal vehicle to the impounded if again a civilian tries to hot wire your vehicle.