URL tokens on multi-site don't properly cache base URL

On multi-site CiviCRM intalls (with both Drupal and WP) clients have reported that automated messages (contribution receipts, PCP setup notifications, etc.) have dropped the base URL from messages. This persisted even after applying CRM-16196. However, I couldn't reliably replicate the problem - apparently because it's a caching issue.

I finally solved the problem by changing this line in CRM_Utils_System::url:
$config = CRM_Core_Config::singleton();

to:
$config = CRM_Core_Config::singleton(TRUE, TRUE);

However, I'm not sure which of those options fixed the problem.

I'm open to troubleshooting further if someone can give me some advice - particularly on how I might best replicate the issue if it IS caching-related.