It seems the Mi Box update released last week that made improvements to Netflix also downgraded YouTube functionality.
Videos watched through a web browser that have a 1080p / 60fps option available will now show 720p as the maximum resolution for 60fps playback within the youtube app on Mi Box.
This is most likely the result of youtube refusing to offer VP9 encoded video to the Mi Box, or the Mi Box not requesting VP9 encoded video. For example, if you disable html5 video playback in the web browser based version of youtube then 1080p / 60fps or any resolution above 1080p is never an option.
Also back, although in a more minor form, is stuttering with 60fps playback in youtube. It depends on the video, but some 720p 60fps videos are freezing for a moment about every 10 to 20 seconds on average for me.
The Mi Box hardware was pretty good at playing back the superior VP9 video at 1080p / 60fps and 4K / 30fps. If it's gone for good then I think I'm done with this product. I'll just bite the bullet and buy a Shield or build the cheapest HTPC I can that is as powerful as the Shield.
They also removed it from My galaxy s5 no clue why it was working fine