That is very strange. I always found it best to disable UPnP, there's just something about automatic port forwarding that is not under my direct control that strikes me as very, very wrong and potentially insecure.
Besides, from what I can tell the P645M-A1 ADSL bridge does not have any UPnP fucntionality anyway? Are you sure its the P645M-A1? Are you configuring it by telnet or with your web browser?