Virgin Media Hub - IPv6 Firewall

Have Virgin Media (cable) broadband in the house, with the Hub 3.0. I want to have external access to a service running locally. I'm on a DS-Lite configuration, but I'm fine with restricting my access to IPv6, everything I plan to use supports it.

However, I can't seem to configure the Hub's firewall to allow specific traffic to the server I want. If I turn off the firewall on the Hub, my service is accessible, but obviously I don't want to have no router-level firewall. So I turn the firewall back on and go to "IP and Port filtering" ⇒ "Create a new rule"

Seems fairly straightforward, so I set the rule up as so (XXXX::003a being my server's IPv6 address, service is listening on port 8000):

Annndddd… nothing. Can't access it.

However, when I set up the rule to allow all ports to that server:

It works 🤔 Again, I'd prefer not to allow all port traffic through to the server.

So has anyone successfully set up an IPv6 firewall rule on the Hub 3.0 for just a single port? Is there something I'm missing with the port range setup?