I do not have IPv6 internet, so I've only been able to test on LAN. I have tested with usernames, port numbers, and zone ids.
It would be great if someone tested with real IPv6 services. There are public servers here, but not all support IPv6. The site cygwin.mirror.rafal.ca might support IPv6 at ftp://[2604:1500:f001:0:216:3eff:fe3f:746b]