NVG589 possible bug in port forwarding?

I'm trying to enable port forwarding (in this case port 3640) for a device on my network, but in the "firewall" "nat/gaming" settings in the gateway config, my device does not show up under "Needed by device".

The device does correctly show up in the "Device List" and it otherwise functions correctly. I can also type the device's IP address in the "passthrough" settings, and achieve my goal, however this leaves the device completely exposed, and I would rather use Port Forwarding.

Is there a way to have the Firewall forward ports by a devices IP address instead of relying on it being detected by the configuration page? Am i misunderstanding how this feature is supposed to work?

Any help is appreciated!