This could be as designed. The user login and admin login are separated. So generally it is possible to login to site as user X, and in the same time to be logged to admin panel as user Y. If you logout from user X (from admin panel) - you are still logged as user Y in the site. But if you log out from user Y (from the site) - it also logout user X in the admin panel. I would expect the logout to affect only the relevant account.