Calling yourdomain.com/preview.php redirects to yourdomain.com/admin/#/site. Calling the url a second time, it will load content but keeps preview.php?/ in its url and you will never be redirected to /admin again.
The above is valid for connections over http and https. You have to be logged out to reproduce the behaviour. Manually deleting API, Site and browser cache leads to a one time redirection to /admin but after a second call it shows the site content again.
Calling yourdomain.com/preview.php redirects to yourdomain.com/admin/#/site. Calling the url a second time, it will load content but keeps preview.php?/ in its url and you will never be redirected to /admin again. The above is valid for connections over http and https. You have to be logged out to reproduce the behaviour. Manually deleting API, Site and browser cache leads to a one time redirection to /admin but after a second call it shows the site content again.