I'm running 3.25. I first enabled upnp and then created two rules internal, external. Didn't work. Spent a couple of hours wondering why it is not working, checked software firewall etc. then found this thread, disabled and enabled upnp once again and now it works. I don't see the dummy rule though.
So, when you create it, it doesn't work if you enable it before you have rules, I guess..
Running 3.25 on my RB450, UPnP we're working on 3.20 but didn't on 3.25.
After disabling / enabling UPnP it works! Dummy rules also!
EDIT: Also need to mention that I'm on a pure ethernet connection with DHCP-Client enabled on WAN interface.