You are right, I have an NUC DC3217IYE running Windows 8.1 Pro.
I have legacy usb already disabled in bios as well as legacy boot.
I also have the IME (Intel Management Engine) driver installed.
DTS-HD and DD+ work fine in Windows, with the Dolby test movie for example and DTS-HD in XBMC.
But DD+ still won't work in the Netflix Metro app.
There are currently 2 ways around this issue:
1. Install the Microsoft display audio driver, then DD+ works but I lose DTS-HD.
2. Start a Netflix series like House of Cards which has DD+, go to the audio device settings and try to set it to stereo.
This will produce an error and then DD+ will work until you stop playback.
At some point before in Windows 8 it has worked, but stopped working after the Windows 8.1 update.
This all point heavily towards the display audio driver in my opinion.