Answers from subzone even when superzone has a delegation elsewhere

[Andy Smith]

Hi,

I'm running:

9.16.44-Debian (Extended Support Version) <id:cd2b460>

If I have zones example.com and sub.example.com both loaded, but
example.com contains a record:

sub.example.com. NS elsewhere.example.com.

(i.e. the subzone is delegated to some other server)

is it normal and expected that a query for foo.sub.example.com
should be answered NXDOMAIN from the auth servers for example.com
because the zone sub.example.com is also loaded there (and has no
"foo" RR), rather than the delegation to elsewhere.example.com be
followed?

If that is expected, is there configuration that can alter that
behaviour, or is that RFC required behaviour that should not be
altered?

Thanks,
Andy