I’m not sure what failed last night, but I’m guessing the cable Internet connection was down. Interestingly, unbound had stopped resolving even local zones configured with stub-zone and stub-addr directives. This was unexpected: stub-zones are supposed to work “without referring to the public Internet” per the unbound.conf manual page.
To mitigate the issue I wanted to have backup name servers in resolv.conf (ones using a different Internet connection) even on the resolving name server hosts themselves. With resolvconf that boiled down to creating /etc/default/resolvconf with the following setting in it:
TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS=no
This way name servers configured in /etc/network/interfaces (using dns-nameservers directives) are included in resolv.conf even when unbound has been started.
However, this is a poor workaround, as I don’t have multiple Internet connections at every site.