The preview release will never update to become a “non” preview release. At a time like this when the version number is the same between the two, then the code between them is exactly the same. But the version is different (one has preview as part of the version name) … and that makes some runtime behavior different, such as where it checks for updates.
More info on preview versions here:
If you’d like to switch to the non preview version just download directly from www.taskpaper.com. Then you’ll only be notified about non-preview versions. If you stick with the preview version you’ll have the same version of the app as todays release version, but you’ll also get future preview updates.