Issue with Amazon claiming I need an FNSKU label

I'm having a problem. I had a recent shipment get sent directly to Amazon and I received a product level defect warning stating that "This product has been set to use Amazon barcodes, which requires that the Amazon-specific FNSKU label be placed over the UPC barcode. For this item, your FNSKU is [XXXXXX] and can be printed from the `Label Products` page when you create a shipment, or at any time from the `Manage Inventory` page. The UPC is only sufficient if the item has been set to use manufacturer barcodes".

Problem with this is, my product is set to utilize manufacturer EAN barcodes, and the labels were prepared and applied to every unit by the manufacturer. So I don't understand what the problem is here. Why is Amazon claiming that I need to use Amazon barcodes when I opted in to use my own? Why is this issue arising?