I have a BIND 9.6 instance that acts as a caching NS for the whole building and is also authoritative for an internal zone ("example" below):

zone "example" {
    type master;
    file "example";
    update-policy { grant dhcp-update subdomain example. A TXT; };
};

Due to a rogue switch we lost connectivity with the rest of the world, and the NS started answering SERVFAIL; what surprised me was that the server was also unable to respond to queries for the example domain.

What is the reason of this behavior? Shouldn't the NS be able to answer since it has authoritative data?

edit: The rest of the configuration is the standard one shipped with Debian: hints for the root servers and the zones for localhost and broadcast.