Hi everyone! i am stucked with the syncronization of google merchant because my shopify said that lots of my products are missing GTIN or MPN even if they CORRECTLY HAVE A BARCODE inside de product sheet.
What should i do in this case? did it happen also to your stores?
thanks for helping 
Yes this is a known issue for over 1.5 years with no fix as of yet. There are several ways forward.
Contact Shopify support and ask them to resolve the issue.
Create a supplement feed to add the missing barcodes: https://support.google.com/merchants/answer/7439058?hl=en
Or what I do for my clients is use the following app as your data feed solution: https://apps.shopify.com/multiple-google-shopping-feeds