This app's backup data is stored in a Google Drive area reserved for app data. That's why the backup file does not appear in the file list in the Google Drive app or in a web browser. This is normal, not a fault.
Open the menu (≡) at the top left of the Notes screen → "Backup" to check the status of your backup ("Updated:" shows when the backup was made). To delete the backup, use "Delete backup" on the same screen.
* The backup data counts toward your Google Drive storage.