DR6300 DNS

We are using NetVault v13.02 with two DR6300 appliances running the latest firmware for on site backup storage.  DNS is hosted by two Windows servers.  Both DR6300's have a static DNS entry defined with both forward and reverse lookup entries.  From time to time we find that one of the DR6300s disappears from the DNS server bringing backup jobs to a halt when clients with active streams can no longer find the storage target.  DNS scavenging would be the obvious answer but it is not enabled on these records.  There hasn't been a problem with the second DR6300.

Has anyone else had seen this happen?