I recently noticed this happening when I found my slave DNS server’s were not resolving any new domains I created. Apparently this “bug” started on a recently update to Webmin, not sure which one.
I have setup webmin’s BIND module to automatically create slave zones on a secondary server through RPC.
The problem is that the slave zone is created with "masters {127.0.0.1;};" in the named.conf file. Thus resulting in the following error: "received notify for zone not authoritative"