Environment: Sitecore 9.1 using Owin middleware
Problem: The user logs into the sitecore content editor and clicks on preview mode from the content editor or the experience editor. The system redirects to the federated login page. Entering any users' details at this point will result in a 404. We would like to reach this page as the default preview authenticated user.
Investigation: The user appears to be entering the page as anonymous and there appears to be no .aspnet.cookies.preview cookie present.