Is there a physics bug with grass and gravel areas outside of the track

Dry grass can provide impressive levels of grip, even more than gravel at times. Wet grass is slippery but, soil moisture content is also a major factor in tire grip / grip-loss on grass. Once the soil has enough moisture, the root system loses its ability to stay in place to the point where the issue quickly compounds. Add to that, loss of grass exposing mud underneath means further loss of grip.

Pooling water and hydro-plaining at speed always poses a danger, no matter what surface it's on. All of these factors combined can make for very dramatic swings in grip conditions on and off track.

Where we live, the water table level can also be a factor in how rain is absorbed into the ground, or not at all. I can't even begin to imagine how challenging this stuff is to program into a racing sim.