I was setting up a new test DC server with Server 2016. When going through the setup a domain controller wizard there was an option for the forest functional level “Windows Server Technical Preview”.
I thought to myself did I install Server with a preview build??
I checked this MS KB article which verified that I wasn’t going crazy and that it’s a bug.
Install the latest CU update to fix this issue (Which as of 23/12/2017 is KB4019472.)