We had the exact same thing happen! Ours looked more like rust spots and cropped up in the first couple of weeks that we had them. I took photos every couple of days to show where the spots were developing, then took them to fabricator, who agreed to replace them. They weren't exactly sure of the cause, but probably iron reacting with water or sealant. We've had the replacements for 2 years and all good- just one tiny rusty spot that's barely noticeable.
Q