I've got 2160p selected & 2160p 24/25/30fps selected to but when I watch Amazon ultra hd content the bar on bottom only every shows hd. On my fire tv stick always loads saying optimizing for ultra hd but on TiVo doesn't do that... bit confused.
I'm amazed this issue hasn't been commented on more. I also have all the correct settings (Netflix 4k plays fine) but all Amazon 4k shows are playing in hd. The top banner says 4k but the actual feed always says HD. I wonder if people just haven't noticed yet.