If you are using the RAW Power Photos Extension (or any extension, for that matter), the extension must send a JPEG (not a HEIF or TIFF) back to Photos. So, if you then export a TIFF from Photos, you will be getting a TIFF made from a JPEG (this is mentioned in the article cited above). Your best bet is to use the Export button in the RAW Power extension, which will make a TIFF that you can store on your local hard disk.