In 2.9 we changed the default setting for the home page to be the "Dashboard" (previously called "My home"). This means admin after login may never hit "Site home" (former "Frontpage"). If upgrade is pending admin is not redirected to the upgrade page because this code is present in /index.php but is missing on /my/index.php :

if ($hassiteconfig && moodle_needs_upgrading()) {

    redirect($CFG->wwwroot .'/'. $CFG->admin .'/index.php');

}