I have been meaning to fix this for a while. I am 99% certain this is caused by the fact that all vehicles on the server are destroyed on city change, but some things 'carry over' from the previous city's vehicles. Two things were not 'reset' when a vehicle is destroyed, which could affect how the vehicle is treated on the next city. This has been fixed which may resolve this issue. I have debug logs which tell me when a dealership vehicle was wrongly destroyed and why, and most of them are from a 'static vehicle respawning' but they are not static vehicles. I think the vehicle IDs were static vehicle IDs in the last city.
I will keep an eye on the logs as the static vehicle respawn is not the only reason it reports.
Another issue was that when the dealership test drive ended/failed, it would destroy your current vehicle and your 'last' vehicle (which would be your current vehicle if things go right). It now explicitly remembers the vehicle which is soley created for the test drive and will only destroy that.