The core global $is_https is not TRUE.

This needs to be added to the settings.php file. See https://api.drupal.org/api/drupal/includes%21bootstrap.inc/function/drup...

if ((isset($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) == 'on')
  || (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
  || (isset($_SERVER['HTTP_HTTPS']) && $_SERVER['HTTP_HTTPS'] == 'on')
) {
  $_SERVER['HTTPS'] = 'on';
}

If you eval this it fails even though it's a https site.

var_export(drupal_is_https());