When testing the AD connection details you need to actually do a search to make sure the credential is valid.

https://github.com/jenkinsci/active-directory-plugin/blob/master/src/main/java/hudson/plugins/active_directory/ActiveDirectorySecurityRealm.java#L392

If in this search there are not elements, we are returning null.

https://github.com/jenkinsci/active-directory-plugin/blob/master/src/main/java/hudson/plugins/active_directory/LDAPSearchBuilder.java#L98

And in this case we are saying that everything is success.

https://github.com/jenkinsci/active-directory-plugin/blob/master/src/main/java/hudson/plugins/active_directory/ActiveDirectorySecurityRealm.java#L425

it might happen that under AD replication farm one of the AD instances is broken not returning any user in the search. In this case, we should expose the issue on the GUI.