Hi,
We faced an issue on our DNS server, actually Client with IP reservation in DHCP doesn't update DNS.
The DNS and DHCP role are set on the same server (2012 R2)
We found this article : https://social.technet.microsoft.com/Forums/windowsserver/en-US/8e972017-2c80-4ea9-a412-d14630a3cd72/client-with-ip-reservation-in-dhcp-doesnt-update-dns-entry-neither-the-ptr-is-that-normal?forum=winserverNIS
They talk about an hot fix, but it seem that can be applied only on 2008 R2.
Regards