Netflix on my MiBox doesnt play with 5.1 surround (no 5.1 icon on the title), Ultra HD it can but not 5.1. I downloaded a test video file from dolby and played it in USB via VLC and the surround kicked in. I think the problem it with Netflix app. This is my setup:
MiBox>>HDMI>>TV(LG)>>HDMI(ARC)>>HomeTheater(Samsung)
Native Netflix app on my TV(LG) plays with 5.1 surround.
Update: I was able to chat with Netflix support, she said that Netflix and xiaomi are working right now to fix issues with MiBox. Nevertheles, she said that she will create a report of my issue to dev team for possible fix.
Your TV probably can't pass through DD+ 5.1 from a HDMI source via ARC.