When clustering two Webmin/Virtualmin Servers together using the Webmin Server Index. When I added the secondary DNS server in using just its IP address.
Then when going to BIND > Cluster Slave Servers, and then manually assigning the servers FQDN (NS2.example.net).
When creating a new Virtual Server, it would add a double dot into the NS section of the zone - for example:
@ IN NS ns1.domain.net. @ IN NS ns2.domain.net..
Notice the two dots?