All custom settings get overwritten and set to Ultra or Extreme on everything.
It may not be making any sort of attempt to "optimize" settings based on my hardware, but that's probably even worse. I turned stuff down to get the framerate that works for me. Just let me keep my setting the way they were before.
I don't even know whether it's Forza changing the settings or Nvidia. But somebody's doing it wrong.
It's probably Nvidia, I've never had Forza reset my graphics settings and Nvidia GeForce experience / the new Nvidia app have a feature where they "optimise" your game specific settings.