That was why I recommended setting your Mac to use a static IP address within your home network; or failing that, at least set up a DHCP reservation in your router for the Mac. That way your Mac will have a stable, unchanging IP address within your home network. And, since your computer will always have the same IP address, you only need to set the port forwarding rule once.
There are many different implementations of UPnP employed across many different routers and software programs, and quite frequently things don't go as one would expect or as they should. Relying on such protocols should really only be a last resort when you can't control other aspects of your network. However, since you can make changes to your router, the best option for you in the long run is to set a manual port forward and a DHCP reservation.