When you load weight (a container for example) onto a car, the tires get flat, but if you have a heavy car the tires can handle the weight. I made an example to illustrate this bug: https://steamcommunity.com/sharedfiles/itemedittext/?id=2595240488
This vehicle is a chassis with the weight of a container.
If you spawn this vehicle the tires get flat.
If you merge the two parts and than spawn it, the tires behave like expected.