I've run into this.
RustRacers don't work on infmap based maps.
If you are spawning vehicle and part of it sinks in.
What you need to do is unspawn the vehicle and probably walk over sections where you plan to spawn the car.
I'm not completely sure but from what it looks like to me what infmap seems to do is cut the map by models and loads them into your game in sections.
You probably spawned a car in between two sections of the map.

I tried to load it right where I appear but they still sink, with the plugin [simfphys] they don't sink and I don't understand why the ones that use the game's vehicle engine sink.

same happening but its the machinecult dupes, idk why the wheels of it sinks in, the fast ones break when it drives and some of them dont move in infmap