I will try to explain as easy as possible:
A samp server has limit of max 2000 vehicles, can confirm from here: Limits - SA-MP Wiki
so after the quest added the number of players increased and more vehicles spawned = vehicle limit being reached and server started confusing vehicle IDs results in mixing them
there is enough free vehicle limit in server so I was wrong about that, this is happening possibly:
while server is spawning and despawning so many vehicles very fast it sometimes confuses vehicle IDs and mixes them with ID of different vehicle from different player/clan