There is debate on what causes it and I personally do not think it is dirt.
My two cents but does not mean I am right.
Exhaust valves are stainless, and intakes are steel with a hard coating on the sealing surface.
Even the exhaust deform, got lots here to look at but they don't do it as much as intake, the difference in metal explains that. Also the fact that most cylinders are just fine with scratching also says it is not the dirt to me.
I do know, installing aftermarket quality valves and stiffer springs fixes it, so if it was dirt then the new ones would do it.
Again this is an opinion and not meant to be argumentive.
Since we have build a good many engines with no issues after the fact with valves I will go with my explanation.
I will also say the stock filter in a Teryx does let dirt pass but the one in the Brute does not and they do this just as bad.
The debate continues!! Who knows, all still Kawi's fault as no other UTV engine does this.