I also have a similar incident where granule is coming of the roof after our cleaning. I have done tens of metal roofs over the years but never had a similar issue. I found out that roofs are Dura Loc and they were installed after hurricane Wilma and were never cleaned before.
The Condo Association is blaming our method of cleaning. We have used light bleach solution (less than 30%) that was sprayed on the roofs without pressure off a boom lift (no walking at all). A couple of weeks later, we received complains that granule is coming off.
The condo has 9 large buildings but we only cleaned 1 building. I have no idea how to rectify this problem and hopping to get some guidelines on what to do here.