If You order an Vehicle, then it is bound to You.
To unbound you have to enter that Vehicle and hold the L Key, if you after unbound the first Vehicle buy a second Vehicle, then the second Vehicle is bound to you. You only can lock/unlock an Vehicle if it is bound to you.
So holding a bit longer the L Key it toggles bound /unbound
if you tip the L Key it toggles lock/unlock.
So in Your Case the first buyed Vehicle was still bound to You and the lock/unlock still was going to the bounded Vehicle.
Holding the L Key a bit longer in your second buyed Vehicle will bound that Vehicle to you and the tip on the L Key (lock/unlock) will now work on the Vehicle you actually entered.