When $g_use_ldap_realname the real name stored in the database should not be used when working with users. Unfortunately, there are places where Mantis does not use user_get_realname() but the direct value of the database field thus leading to a potential problem if those are different.
There is an example of this in manage_user_page.php at line 308 (1.2.0 release)
Another example if the user_get_id_by_realname() function in user_api.php
There are many others around Mantis, just do a search for "realname". Not all occurrences are easily fixable

I understand that this slows downs processes, but this is required as the real name in the LDAP structures may change after the user has been added in Mantis (a wedding is an example of a situation where this occurs).