In some circumstances, HQ does not correctly display currently running Agents after it has restarted.
Before HQ Restart
After HQ Restart
Agent display is blank.
Workaround
To restore the Agent list:
On the Agents page, hover over the icon for the blank Agent to display the Edit (pencil) icon
Click the Edit icon to display the text entry box
Click the check mark (no entry is needed)
Reload the Agents page