The question is not new: how could could gold corrode? I confirmed the issue with Berlin. There was some kind of substrate material used to bond the gold to the Mylar. That material over time had an electro-chemical reaction causing the corona oxidization.
I've observed and dealt with the K47/67/87 corona defects since the late 1980s and have not found the issue getting worse with time in most cases; rather, the deterioration seems to arrest at some point, unless accelerated by environmental factors.