We have 2 DHCP servers, both setup with the whole scope, but each with exclusions so they give out half the scope each. They are also both DNS servers, so each gives itself out as the primary DNS and the other server as secondary. A basic load balancing of sorts.
This has always worked fine.

One of them is being retired as I have a new server to replace it. Over the holidays I have added it as an additional DNS server, then setup DHCP, before disabling and deauthorizing the old server and authorizing the new. So its supposed to do exactly the same, only difference is that its on a different IP and thus gives out a different DNS server.

I powered on some machines that I saw had leases on the old server, and sure enough they sent out requests and got a new IP address. Most from the new server. All working fine, except the DNS server addresses are not right. They're not even showing as what was setup on the old server (which I could have explained away) they are instead showing the config that the other server should give out - eg instead of 172.16.1.1, 172.16.1.2, its 172.16.1.2, 172.16.1.1

If I do ipconfig/renew then everything changes to what it should be. But then, rebooting the machine, it comes back up with the wrong settings again!

This isnt a massive problem. since whichever way the DNS servers are listed, both are active. But I'm interested/concerned as to why its doing this. Any ideas?