When you install Server 2016 then try to promote it as Domain Controller, Forest and Domain Functional Level come to “Technical Preview” but that’s not correct cause this is a Server 2016 RTM version.
When you start to install updates to Server 2016, then try to promote it as DC and it gets correct FFL & DFL .
The trick is you should fully patch Windows Server 2016 first then promote as Domain Controller.(Also need to install following Cumulative Update)
Also check this “Cumulative Update for Windows Server 2016: September 26, 2016”