So, it turns out, that if you want a dual stack OpenBSD box, operating as a relayd proxy, to be able to talk to another server on the same VLAN using IPv6, you kinda need some pf firewall rules to allow inbound IPv6 stuff for it to work. You know, stuff like neighbour discovery works much better when the packets can actually arrive at the interface.
I’ve been fighting this weirdness, on and off, for months and in the end just used legacy IP for the back haul instead. I had an epiphany just now while adding relayd config for another backend server in a different VLAN that just worked over IPv6.
I was always convinced it was a “me” problem, as nobody else seemed to have issues. Turns out I was just looking in the wrong place (my relayd config) all along.