You also have the logic backwards. An item name is substituted in the description field only when the description is blank. The description does not really replace the item name. The difference may seem subtle, but is important for understanding the purpose of the fields. The description should contain whatever you want to show on finished documents. This is clearly explained in the Guide: https://www.manager.io/guides/7551.
And this will prefill your line description with item name and from there you can add additional descriptions in the Inventory item edit form or add additional line comments as you go when creating your invoice.