The issue is the plastic materials that BMW uses for these rubber gaskets. It is UV that is damaging the rubber.
Not sure about the painting options, but I know that in general UV is a problem.
This is a persistent problem on Z4's and I have discussed with a dealer who advised they always have customers complaining a couple years into ownership. According to the stealer, BMW uses "green" products which is why these deteriorate quicker. I think that's a load of
-R