The upnp functionality seems to be quiet buggy.
If I delete all dynamic firewall rules and restart Plex, it maps itself a port successfully.
If I restart Plex, it attempts to remap the port and FAILS. The router shows the same firewall rules, but now the port is no longer being forwarded correctly. I have to manually remove the rule and restart plex again to get it to work.
This is also affecting the Xbox as well, as the port forwarding varies throughout the day.
This seems like a rather large issue…