Does that computer use network manager? Or static config?
That one uses wicked. I have dhcpv4 and dhcpv6 and radvd.
So, that would be the same as my desktop system, which uses the IPv6
DNS server. However, my ThinkPad, which uses the network manager
uses IPv4, though the same computer uses IPv6 when running Windows.
My Android phone also uses IPv6. So, it appears there is a problem
with network manager not using IPv6 DNS servers, whether with RDNSS
or DHCPv6. I guess I'll have to file a bug report.