Login redirect is not working/wrong with Shibboleth

I have a page at this location: https://site1.wp.domain.com/activate which requires log on. The user click on the Login link and is taken to https://site1.wp.domain.com/wp-login.php?redirect_to=%2Factivate%2F

If I login as a local user using the WP login box, everything is fine – I get redirected to the activate page. However, I have a Login with Shibboleth link, and if I login with this, it takes me to the main site, not site1 (obviously complaining that the page does not exist.)

The Shibboleth link is: https://site1.wp.domain.com/wp-login.php?redirect_to=%2Factivate%2F&action=shibboleth (hovering over the Login with Shibboleth link.)

How do I get the redirect to work properly? Obviously if I manually replace the string ‘%2Factivate%2F’ with ‘https://site1.wp.domaim.com/activate’ it works, but how do I do it automatically?

The Shibboleth plugin code for redirect is:

// Add subdomain to redirect allow list for multisite installations
$allow_sitewide_redirects = shibboleth_get_option('shibboleth_allow_sitewide_redirects');
if ($allow_sitewide_redirects) {
       add_filter('allowed_redirect_hosts' , 'shibboleth_allow_redirect_to_subdomain' , 10);
       function shibboleth_allow_redirect_to_subdomain($allowed_hosts) {
               if (empty($_REQUEST['redirect_to'])) {
                       return $allowed_hosts;
               }

               $redirect_url = parse_url($_REQUEST['redirect_to']);
               $network_home_url = parse_url(network_home_url());
               if ($redirect_url['host'] === $network_home_url['host']) {
                       return $allowed_hosts;
               }

               $pos = strpos($redirect_url['host'], '.');
               if (($pos !== false) && (substr($redirect_url['host'], $pos + 1) === $network_home_url['ho
st'])) {
                       $allowed_hosts[] = $redirect_url['host'];
               }

               return $allowed_hosts;
       }
}

Anyone know what to change to get it to work correctly?