The only best 4K HDR content streaming set-top box is the Nvidia Shield TV. However, due to some issues in Prime Video, users were not able to play 4K content on Prime Video, and a recent update fixed that issue.
Nvidia confirms that Amazon is releasing an update to fix the issue of 4K content. Nvidia Shield TV, especially the Shield TV Pro from 2019 has this issue. Instead of playing 4K HDR content to the fullest, the bugs caused the videos to play on 1080p SDR instead.
Although issue is unclear, Amazon has finally found the workaround and is now releasing the fix for the same. As per some users, the app update is getting the job done already.
If you are one of the Shield TV owners, you will get the app update within 3 days.