I have had this happen before and support assisted in fixing this but this time around they say supported has ended and I have to post here.
I have a number of static entries in DNS for servers located on the other side of a Netvanta to Netvanta VPN. This has worked just fine for the longest time however twice this week an entry for one server has disappeared from the DNS entry list. Not sure what else I can say about that. The entry was there for month, it disappeared, I re-entered it, it disappeared two days later.