Thought I'd mention I'm experiencing this as well. Have an ISP with native IPv6 through DHCPv6+PD… the WAN interface would get its address (it shows in the interface status page), but the LAN wouldn't, and the IPv6 gateway always showed as down. This happens whether I request a /64 or a /60 from my ISP.
As another data point, this worked fine in the latest 2.1 release version on a different computer... I had to use the 2.2 alpha on my current system due to the new version of FreeBSD resolving a kernel panic issue with my new hardware.
I was able to set up a GIF tunnel with static IPv6 on the LAN and it worked just fine, though. So it seems to be something about IPv6 and DHCP.
EDIT: My issue was not related to this; I had a stray IPv6 tunnel gateway left from importing my 2.1.x config. After removing it, IPv6 with DHCP+PD from my ISP is working properly.