I think I found the issue but how to work around it is another question
Our forwarded lookup zone file is 4Mb in size and has 1956788 records (A, AAAA and CName)
If I delete loads of records from this file and get it below 2Mb all works with no issue but as soon as I go over the 2Mb the issue starts
Is there a limitation on the size of any one zone or is anyone aware of a workaround for lager zones
SolarWinds has confirmed the 2Mb limitation on Zone files
Each zone must be smaller than 2MB or the session will time out before it can add a record to the DNS server
quote "reducing the DNS file zone size below 2mb is the only way that we can make it work with IPAM"