I don't think it's a defect but a logic characteristic of the game. The bodywork is not very important for the functionality of a vehicle, as only the external part can be damaged and not the chassis. The important things in this game are the wheels, the battery, the spark plug, and the radiator ... all of which you can replace. Then there is the engine, which is fundamental. This alone should be a condition for deciding whether a vehicle should disappear or remain: a destroyed and irreparable engine renders a vehicle useless and it is right for it to disappear. If the engine is still repairable, if the bodywork is all broken and accessories are missing, you can always repair the engine, add all the accessories and play with the broken bodywork: the vehicle is still usable and it is not right or logical for it to disappear. .
My 5 cents

The vehicle cannot be driven if the body is ruined, FYI.

You haven't read what I wrote.
Read it better.
The external sheet metal has no necessary function other than the aesthetic one, therefore it is of no use to anything essential, the vehicles can move without problems. This game does not foresee damage to the chassis (which is essential), but only to the external bodywork.
Get over it, this is it.

You might be brain damaged. The vehicle is no longer functional, can't be repaired, can't be driven, and can't be despawned.

Meanwhile, there is no need to insult!
I don't know if it's a Google translation problem (I don't know English), but from my experience with the broken bodywork you can drive normally. I do it regularly with the old Skoda and with a new Niva. Maybe because it's not ALL broken, but it has damaged parts that cannot be repaired with epoxy resin ... and I continue to drive them.
And I repeat that this is right.
If you want to make a car disappear, take out all the water, start the engine and let it go until it breaks down. When the engine light turns red, that car will be ready to disappear after a short time. I've seen it several times.
Maybe, if you want to be safer, remove all the wheels and body parts as well. With the engine melted it disappears for sure.

@Geez try repeatedly wrecking at low speeds. Eventually, the body becomes ruined but the engine is pristine.