The source – support.google.com
Beginning in August, Google Merchant Center will begin displaying "Invalid Product ID [mpn]" notifications. They will be shown in the account on the "Diagnostics" page for advertisers who set an incorrect value for the MPN attribute for one or more products. And from September 2021, products for which warnings have been received will be automatically rejected. To avoid this, advertisers will need to provide correct product data and remove the incorrect MPN attribute value.