Hello!
With dual ipv4 + ipv6 connection transmission does not handle port forwarding correctly. ipv4 done using upnp, buf if here is a ipv6 connection transmission can not tell if port is opened correctly.
When you hit 'check port' transmission says port is closed, when you drop ipv6 - it says port is open.
ipv6 can be done two ways - public ip, or nat. transmission should undertstand it.
when 'port check' hit, transmission should use ipv4 connection to test upnp ipv4 mappings, and separate test for ipv6 connection for booth ipv6 public, and ipv6 nat.