If I have a server which has an IPv6 interface but it is not connected to the internet (eg a default docker container or most linux servers these days), the first time I run a query for 04u-ua5db72ef-c12-a2093-s1613236523-i52dab726-0.eu.dotnxdomain.net it returns a SERVFAIL. If I set prefer-ip4: yes it fails similarly. The only way to make this resolve correctly the first time is to do-ip6: no.

From what I can see the difference is if there is ipv6 enabled we get this log line repeated a few times (even if prefer-ip4: yes for some reason):

[1613241220] unbound[1:0] error: udp connect failed: Cannot assign requested address for 2a00:ab00:603:96::32 port 53
[1613241220] unbound[1:0] info: error sending query to auth server 2a00:ab00:603:96::32 port 53

It also tries to do IPv6 queries even when there is only an ipv4 address listed in outgoing-interface.

Unbound 1.13.0