The key phrase here is "Diamond cut". The alloy surface is polished to a mirror finish then given several coats of clear lacquer. The effect is stunning, but not hard wearing. The mirror finish gives the lacquer nothing as a "key" to adhere to, so that it is easily chipped off by stone impacts and by tyre fitters replacing the tyres. Painted alloys are not polished as finely, so their surface has a "key" to enable the paint to cling more effectively. In addition, lacquer is more brittle than paint.
This is why alloy wheel refurbishers will give you a 12-month warranty on painted alloys, but only 6 months on alloys with a diamond cut finish.
I had diamond cut alloys on my last motor, a Mondeo Titanium X, and settled for a painted finish because so many Mondeo ST drivers had trouble with corroded diamond-cut alloys.
So I guess that Audi might replace or refurbish them for you once, but they will cause an ongoing problem for you and for Audi. My advice is to get them painted or powder coated and remove the source of the problem.