After extensive testing (including monitoring network requests, reverse-engineering Facebook’s GraphQL endpoints), for more than a few days. Facebook updates its API frequently.
When a user downloads an "Anonymous Story Viewer" app, they are often greeted with a human verification step ("Complete a survey to view the Story") or a request for permissions that have nothing to do with viewing media. These apps are often data-harvesting operations. They don't actually let you view the Story; they simply wanted access to your contact list or ad data. facebook story viewer anonymous