ICMPv6 types 133 through 137
Looks like blocking ICMPv6 types 133 through 137 on the public tunnel is one way to "fix" this. My firewall rules have been updated.
So far it hasn't seemed to affect my ability to access anything via IPv6. I'll know soon enough I guess.
protocol described (briefly) here:
https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol