Update: As LeftNoRightTurn discovered - there's an un-notified Netflix update in the playstore - installing this immediately resolved the issue for me \o/
Got my Shield TV today, to replace my gen1 AFTV. It's amazing as expected. Got an oddity tho, cant get Netflix to do HDR. Amazon Prime does, but Netflix just lists content that I know should be in HDR as UltraHD 4k. I double checked by playing some of this content (e.g. Jessica Jones) and whilst its in 4k, its not in HDR.
Standalone HDR files play via the video player from USB, and in SPMC (after some fiddling with settings) so I'm not sure what's up. I'm on the proper subscription level and playback settings with Netflix.
I'm quoting this from the Shield forum as something you can try.