Currently, bb-admin/admin-header.php uses active template's logged-in.php. I changed this php in my theme, and it looks a little bit strange in admin page.
I think admin page shouldn't use any files from templates, even the default template. Could this be an issue?
And it seems no need to use login_form(), it should just include a logged-in.php in bb-admin or simply put codes within admin-header.php.