As Confluence does not know about user visibilities, there was an issue mentioning users with common names in Confluence versions prior 6.7. This has been fixed in the current release and therefore mentions are working as expected again.
Reporting an issue is sometimes hard
For reproducing an issue we need as much information as we can get. That is why we added the possibility to create a support file including most of the Space Privacy settings you took. This helps us supporting you faster!
Version 2.4.1 - The cleanup space admins release
Removing an extranet space manager
An extranet space manager is a user manager with space administrator permissions. These permissions will be removed if the extranet space manager is unassigned from an extranet.