Every time I boot into a game with the new materials; they seem completely missing when I enable this FastFlag. I mainly have it enabled to have it early since I use it on the Studio build all the time.
It stays like this until you reinstall or update Roblox. You cannot return it by simply turning the FastFlag off. It does not affect Roblox Studio with the new experience controls beta enabled.
Expected behaviour
It should have all the new materials available no matter what.