I think I’ve managed to narrow down my issue at https://www.reddit.com/r/AndroidTV/comments/8ty2ig/netflix_51_apparently_officially_unsupported_on/?st=JJ05DVMZ&sh=da5007a6 to Samsung (and possibly Yamaha) sound bars.
I’ve discovered that when the Shield was updated to Nougat, users with the same sound bar as me (HW-K950) experienced the exact same problem.
More detailed findings here: https://xiaomi.eu/community/threads/xiaomi-mdz-16-ab-oreo-update-build-2116-received.44396/page-10#post-424317
For the record, Xiaomi support (or affiliated) has been terrible.
I’m awaiting a follow up call from Netflix this evening, who are my last hope at this point (not counting on Xiaomi). It could be that Nvidia, ironically, end up saving the day though (see detailed findings).
I have a Yamaha Sound Projector (YSP-1600) hooked up to my TV via ARC (Mi Box is connected to the TV via HDMI) and with Oreo I get Dolby Digital just fine. I don't know about DD+ because I don't think my YSP supports it, but no issues with regular 5.1 DD surround.