In your google merchant center feed rules, do not use set to, instead use modification option clear.
Also makes sure you clear the brand value.
And mpn value.
The below is what I recommend and additional information:
When submitting product identifiers, the following should be applied.
If a manufacturer has assigned a bar code, then submit brand, MPN (if available), GTIN, and set identifier exists to yes.
If the manufacturer has not assigned a bar code, then remove the values brand, MPN, GTIN, and set identifier exists to no.
While the official documentation does mention, you can submit brand + MPN and ignore GTIN. However, in most cases, this will cause issues and generate warnings such as limited performance due to missing identifiers.
And as of June 2021, specific categories and or countries require barcodes to be submitted, regardless if you have any. So, in this case, you need to get official GS1 barcodes.
Reference: https://support.google.com/merchants/answer/6098295
A secondary reason for issues is that most merchants tend to use SKU as the MPN values, which is incorrect.
SKU stands for “stock keeping unit” and — as the name suggests — it is a number (usually eight alphanumeric digits) that retailers assign to products to keep track of stock levels internally.
An MPN, or manufacturer part number, is a manufacturer-assigned, unique, alphanumeric value used to identify a product among other products from the same manufacturer.
More info:
