I just tested this on my end and I do not have this issue. I know what to look for as I did have this issue a while back and then they did an update that fixed it. Back then the issue was with Batman vs. Superman where it would exactly like your top screen shot and I also had a Samsung UHD player that looked like your bottom pic. I just tested BvsS on my Sony 4k 940d with my Wife's just updated system and the black bars where pitch black, so issue on my end. When I get a chance I will take it upstairs and try it on my new LG OLED to see how it looks. But when I had this issue a while back, it was with the same Sony 940d that I was having the issue with. I don't really remember if it was happening with other movies, but it was enough just to have it with BvS and it was irritating.
Regardless of my results, I hope they fix it. Which movie are you testing with? I want to see if I have the same movie so I can do an Apples to Apples using the same movie to ensure that I don't have this problem.
My wife, which I just updated her system loves that the dash is so much faster and she loved setting up her new home screens. She also requested I set up her external with all her games and that I checked off the Xbox One X updates. She is not a huge gamer and before we met she rarely ever played a game, but she digs playing some on-line MP games with me, ESO, Division and she even beat Diablo 3, which I have yet to do myself. Anyways, she loves the new dash and prefers the bright (360 esque) look instead of the Dark.