My product feed was rejected today. How can I investigate the combination of GTIN and image CDN issues?

Google Merchant Center suddenly rejected a batch of products today, showing a mix of identifier and image issues. The operations team found the feed error logs confusing, so I exported the rejected SKUs and grouped them by GTIN, brand, MPN, and image URL response code. I eventually discovered that some variants were reusing the parent SKU's GTIN, while other image CDNs were returning 403 errors…