If each ingot was turned into a ring then it does indicate a mistake unless one is just gold-plated. There isn't much silver-colored metal on Vilya. It is possible they just had a extra ingot or left over material I guess. It could be something Sauron steals in Season 2 or used to create the other rings. It isn't a big deal that a mistake like that exists but it is still a continuity goof for the show as far as we know. It being a mistake has nothing to do with Tolkien's work.