I had this missing textures thing happening in Demo on Dubai track. Looks like the demo was pretty accurate about, what is going to be in the full game, including problems.
Did you set the “resolution scale” in forza graphics settings above 100% ? do you run 1080P or 4K?
Try to go in nvidia control panel - power management mode - prefer high performance
Then in regular windows control panel - power options - high performance plan
idk if these things will work, but it’s worth a shot
Well i had my PC at High performance before , i don’t know how it went back to balanced , but it did fix the Missing Ground issue . The game is scaled to 100%
The textures on the Subaru Rim is still there . i’m guessing that’s a Game bug .