Sometimes it can take up to two minutes until objects created via createVehicle server side to be visible on clients. Clients do not know the Net ID of the spawned vehicles during that time. Vehicles always show up, but it can take a loooooong time for them do so time. This is very critical if you need to do cargo operations, as they are out of sync.
Confirmed in 1.48, 1.50, 1.52 RC and 1.53 DEV. Only tested for JIP.