I’m in the Alpha Ring for the xbox Insiders. The latest update, 08/07, brought a new UI to the xbox. It also brought grey borders to the top and bottom of 1080 videos.
It goes without saying, it really sucks. I checked Netflix, and the 1080 streams there are still black on the letterbox borders.
Picture aint the greatest in the low light, but you can see the grey on the bottom part pretty well.