I’ve never seen painted keycaps that -don’t- get shiny with use. (It’s happened to all of my macbooks)
That doesn’t mean the finish is wearing off. (The underlayer of paint is matte and would be flaking off if the UV protectant coat was actually being removed.)
As a side note, you should be cleaning the keycaps with nothing harsher than water. Alcohol and other solvents will destroy the UV-protectant coat -and- the paint.