ndp for ND (ipv6) proxying on /64 prefix is failing cryptically.

Hello, everyone

I am running an OpenBSD 6.9 Vultr node. Vultr is issuing /64 prefixes with
SLAAC. I have a few machines behind this node, connected via wireguard.

For simplicity, let us say that vio0 is the default interface,
configured the way Vultr suggests:

hostname.vio0
dhcp
inet6 autoconf -temporary -soii

wireguard is configured like this:
hostname.wg0
inet6 <a-static-ip-from-vultr-prefix> 128
!/usr/local/bin/wg setconf wg0 /etc/wireguard/server.conf

from the outside I cannot pint <a-static-ip-from-vultr-prefix> ,
the response being
2401:c080:1c00:a4::33 icmp_seq=3 Destination unreachable: Address unreachable

If my understanding is correct, that is because wg0 cannot respond
to ND requests from the router.

Trying to set up proxy NDP, I am running ndp like thi:

ndp -s <a-static-ip-from-vultr-prefix> <vio0-mac-address> temp proxy

If I understand correctly, this should make "vio0" announce the static
ip to the router. (the words "temp" and "proxy" seem to have no effect)

However, ndp errs:

ndp: set: cannot configure a new entry

There seems to have been a similar error in 2008:
http://openbsd-archive.7691.n7.nabble.com/error-with-ndp-only-on-sparc64-td200752.html
and
https://marc.info/?l=openbsd-ipv6&m=120731349004033&w=2

sysctl:
net.inet6.ip6.forwarding=1
but
net.inet6.ip6.accept_rtadv=0
seems to have disappeared.

What is it that I am doing wrong?
The nabble message mentions some bug in ndp, but it should have
disappeared long ago.

Perhaps, an FAQ entry related to ndp would be nice to have?
Ipv6 is supposed to have the adoption rate of 33%, not such an uncommon
thing any more.

--
Your sincerely,
Vladimir Nikishkin (MiEr, lockywolf)
(Laptop)