Diamond cut/laser cut/machine cut alloys are a PITA. It's a given that some of them will have such problems: corrosion and 'white worm' under the lacquer. It's for this reason, amongst many, that I went for 400 18" painted alloys. Easier to fix, easy to insure, tougher, longer lasting, better ride, less noise, better steering feel, better handling, less expensive though not quite as pretty. Beauty has its disadvantages.
BMW should replace them under warranty ...... hopefully. How very annoying. Best of luck.