You may have noticed that since deploying version 5.8 and its hotfixes, helpdesk users can no longer freely search for users if Selfservice site is configured to prevent searches.
Helpdesk users are automatically limited to the specified attribute (default: sAMAccountName).
This is a product defect (VSTS101164). The product should provide you the alternative to enforce the setting individually for each site: Selfservice and Helpdesk.
The issue is introduced if Hotfix261449 and Hotfix261899 have been deployed to the PM server. If you are/were not facing any login issues, alternatively uninstall both hotfixes the original helpdesk search behaviour will be restored.
This will be fixed on a future release of the product Please refer to this article for updates or contact support referencing the defect ID VSTS101164.