When a vehicle's wheels are destroyed, the corpses of its crews, will be dropped by the player, or other ai-unit underground.
Video example 1 here - https://youtu.be/x01ZMOvCC6k
Video example 2 here - https://youtu.be/iG6vaspJBLQ
Video example 3 here - https://youtu.be/9M7TQ9nCPEE
I suspect that , without wheels the Root Point of the destroyed vehicle goes down,
for the this reason, the crew corpses will be spawned underground.
Please raise higherthe spawn point for dead crews