Are you using Infuse or also another player to play your UHD ISO or remuxes on the Apple TV?
- Firecore plans to address HDR issues for the next update 5.6.8, so probably in a few weeks. See here: Upcoming Features (updated 4/28/25)
- Since there is also more than probably a tvOS issue, it is in the hands of Apple and I have no idea when they will solve it. If you have an Apple Developer account, you can submit a bug report (I did) on https://bugreport.apple.com . Otherwise, you can submit feedback on the Apple feedback page, which could speed things up. It’s here: Feedback - Apple TV - Apple
Have a great day 