This isn't really a bug. There is a timer which checks if anyone is in the area. If so, he will be respawned somewhere else. This timer runs only every couple of seconds. We can increase that, but that will also increase lag because the gamemode will be heavier. The gamemode is heavy enough already 
It's not really priority to change this because we think noone will be harmed if anyone steals a car from the derby arena. When someone starts the derby the cars will be respawned anyway.
It's not really priority to change this because we think noone will be harmed if anyone steals a car from the derby arena. When someone starts the derby the cars will be respawned anyway.