Thanks for your message.
This is because for existing deliveries, the app already created a partial delivery with the provider set to "Unknown". To test the integration for old orders, you'd need to delete that partial delivery first and refresh afterwards. Because of the the way the Amazon integration works, the more detailed status messages will appear below the default Amazon status messages though so you might need to scroll down (again this is only the case orders that are tracked after they have been shipped instead of being continuously refreshed after placing the order).