Outgoing master server connections do not honor net_address_ipv6
xonotic-dedicated is configured to use net_address_ipv6 [something::666] The default v6 ip of the server is [something::2] The server is configured to prefer IPv6 oder IPv4 in general, xonotic also listens on a v4 address.
xonotic itself listens fine on [something::666] but the connections to the master servers (sv_masterextra*) xonotic will make to announce itself go out on [something::2] and not on net_address_ipv6. The master servers reply on [something::2] where of course xonotic does not listen, resulting in the xonotic server not getting added to the master.
Maybe related, in XonStat it seems there is the same problem or a similar one, the stat page for the server also shows ::2 and not ::666 (net_address_ipv6).
Just tested v4 too: The same problem applies to IPv4 when using a net_address which differs from the servers default IPv4 address.