Maybe that is one of the dreaded soy plastics that seem to keep breaking down, more or less, no matter what's done.
A couple of days ago, I heard an ocean scientist talking about modern plastic and how water creatures get sick from eating it. He noted that we have known how to make more degradable soy-based plastic since the 1930s.
"Damn straight," I said, "and I've got the rotting radio knobs in my workshop to prove it!"
Your knobs still have the original shape. As a last resort, if they continue to deteriorate, you could use one to make a model and then cast acrylic repros.
Phil