If you don’t have a script that respawns vehicles when needed, then they will just despawn. Not sure what you are using that it respawns them after a restart.
Spawning vehicles on client side won’t make them persistent. Even if you spawn the vehicles server side using CreateVehicleServerSetter it is still no guarantee that they will just stay indefinitely.
I do offer a script (paid) which will respawn vehicles accordingly: AdvancedParking v3.13.0 - Persistent vehicles! [ESX] [QB] [Standalone]
There are others out there (even free ones) but of course I do not know how they work exactly. Just look for parking in the release section 