pptp does not use User Manager
In the wiki (http://doc.pfsense.org/index.php/PPTP_VPN_Settings and http://doc.pfsense.org/index.php/User_Manager) it says that pptp users should be determined by the main user manager instead of the separate one in the pptp settings. As of the snapshot on December 13th, it is still using the old user manager.
#5 Updated by Ermal Luçi over 8 years ago
The only way to solve this is teach mpd about external script used to authenticate users, as for OpenVPN/IPSec etc.
MPD already supports external script authentication and the existing OpenVPN script just needs to be tuned to work with how MPD sends parameters and recevies the status back.