I think I might know the answer to your problem.
It looks like your loop-the-loop geometry is very thin at the top. I suspect that the vehicle raycast is picking up the normal pointing upwards on the outside of the loop instead of the normal pointing downwards on the inside of the loop. This will leave the car hanging, just as you are seeing.
To prove this you can inspect the normals recorded by the vehicle with the function PxVehicleWheelsDynData::getTireDrivableSurfaceContactNormal. If the normal ever points upwards when the car is at the peak of the loop then we have found the answer to the gravity-defying car. I suspect that the car sometimes picks up the correct normal and sometimes the wrong normal because the car does fall downwards for a few frames in a row before being pushed upwards again.
The solution here is to thicken the geometry so that the inside and outside faces are at least a few centimetres apart. Can you try this out?