The beta versions have a different URL, e.g. editor.construct.net/r92 for the r92 beta. The stable release runs on editor.construct.net. If you bookmark/add to homescreen/otherwise load editor.construct.net, you're loading the latest stable release (currently r89). Then if you've opted in to get notifications about beta releases, you'll see a notification.
So currently it's all working as intended, but this question has come up a few times now, so I guess we ought to tweak it to make it better. Maybe stable releases should only prompt you about a new beta once?