I'm sure someone else reported a display issue with an MX box and a new 4K TV in AF, Android Questions. It might have been a Panasonic or a Vizio, but apparently it was OK with their older TVs but not the new 4K UHD one. Which I'm sure was a bug with the box itself, and nothing could be done really apart from doing an RMA as defective, and trying a different box. Given what these things are, cheapo devices from anonymous manufacturers, likely it was never tested properly for 4K compatibility during R&D or QA at the factory. These things are often faulty, buggy or simply DOA anyway.