If im not mistaken, the pptp server relies on ISC dhcpd to hand out ip's.
Generally with ISC dhcpd, if the ip's are within the scope of the dhcp range, static ip assignment to the same machine over time is not always guarenteed.
This is mentioned here http://doc.pfsense.com/index.php/Why_can%27t_I_have_static_mappings_inside_my_DHCP_range%3F
Although it does not mention pptp server per se, I think this is what youre experiencing with your setup.
I could be completetly wrong.
Just my 2cent