rndc flushname not working

[Frank Bulk]

Our ISP operations are running a mixture of 9.7.3 and 9.8.4 on several
Debian servers and we've noticed that rndc flushname doesn't work many
times.

This weekend we had a local institution whose own authoritative DNS servers
[all of them] were offline for 48+ hours and so there were several
negatively cached entries in our resolvers' caches from customers who were
querying for their institutions website and MX record.  After the
institution restored connectivity to their DNS servers this afternoon I
checked all our resolvers and a few returned NXDOMAIN for the www and MX
record.  Issuing rndc flushname <domain> and rndc flushname <www.domain>
didn't clear out the NXDOMAIN.  I had to use "rndc flush" to resolve the
issue.

Is this expected behavior?  The next time I see what, what troubleshoot
steps should I take diagnose the issue?  Dump the DB?

Frank