Well it shouldnt do but however not everything works flawlessly. UPnP does not always work as it should and i have found this to be the case many times when embracing this protocol using different applications.

I recall this problem sharing a router with a friend while both running DC++ using the UPnP protocol. It worked for a while but upon restarting the application i found his worked and mine didnt. Or sometimes the other way around.

Best thing to do would be to manually forward ports on your router. Once it is setup correctly, it is pretty much guaranteed to work as the network traffic is assigned to specific ports.

Where as with UPnP the router selects the ports for the traffic to flow and therefore is a small probability for any inconsistencies, heavily depending on the router's firmware.

Also firmware updates are essential.