Changed WAN prefix delegation from none to 64 and noticed my browsing was sluggish. Back when I was using a HE tunnel I would notice that if my IPv6 connectivity went down for some reason (browsers tend to prefer IPv6 IIRC and it would take time to fail out IPv6 and go back to IPv4). I refreshed the Google test page and it says I have IPv6 connectivity. Ran a ping test and still no go.
Reset the WAN interface PD to None to try and recreate the old settings, now it doesn't matter if I set it to None or 64 but I still can't get IPv6 connectivity (though it appears my browsing speed has returned to normal).
Looks like it was partially working for less than a minute. I don't know if me changing the PD setting fixed it this time or not (and if it did, why didn't setting it back to 64 "re-fix" it?).