Alright the issue is that when using Lot numbers, the human readable numbers that would normally be under the barcode, are replaced.

How do we get that back?

UPC = Universal Product Code.  GTIN = Global trade identification number.  These show the same information, just in different barcode format.  But in additon GTIN is part of the GS1 system. Take a loook at https://www.gs1.org/  to get more information.  (I am not so familiar with UPC code. I Think they are mostly used in the US)

And both of these represent your SKU = Stock keeping unit.  In Odoo you save the product id for UPC or GTIN in the field Barcode on the product.

Lot numbers got nothing directly to do with GTIN.  GTIN indentify your product. Lot number identify a qty of the product in stock, and give you traceability of the lot origin and all transactions used for it in inventory.