Wow that was a proper investigation
Gemandpearlover!
Unfortunately, no coating is permanent and while some might last long - most won't. The grade of pearls you actually happen to buy is below commercial grade - it's a mixture of cultured pearls with no luster that are later covered in artificial substances (like fish scale paint) to improve the look. There are numerous solutions that can be used for this purpose, but even when using proper ones (that are used on well known commercial brands of artificial glass pearls) - the fact that you have real cultured pearl under it doesn't allow proper bond between the substances and natural imperfections create weak points that over (quick) time become places where the coating peels off. Getting natural substances trapped between the actual pearl and the artificial layer contributes to the color change.
All this said, unfortunately, some vendors call these pearls "real cultured" while in actual wearability these pearls are far below the quality of either low-grade cultured pearls without coating or artificial glass pearls. I would have started the dispute case through etsy to get your money back and wouldn't talk to this vendor any more as they seem not to care about your satisfaction. I'm sorry you had bad experience - but you surely can be calm about your new pearls from Wendy
enjoy!