From what I understand from Exile's code, damage immunity is only applied to player vehicles if the player enters the safezone while inside the vehicle. However, in other situations, such as on a server start, if player vehicles are spawned within the safezone, no immunity is applied. The player must enter the vehicle, exit the safezone, and re-enter in order to properly apply damage immunity on the vehicle. I do not know if this is functioning as intended to deter parking vehicles in safezones, but just a heads up to the devs.