Hi Tiago
I cloned a users espace and took a look at the SynchronizeActiveDirectoryUser action. It checks a site property called SyncExternalAuthenticationUsersActiveStatus before it set's the is_active flag.
If set to false, users will not be set as inactive when they are inactive in the external authentication system.
It's set to true by default.
However I cannot find any site properties for the published Users espace. Do you have any site properties for the Users espace in your environment?
Fabian