I don't know why this topic is marked as "Solved" because this issue certainly is not fixed!
I am having the same issue on my Roku Streaming Stick 4K putting out extremely high contrast and red color saturation when Streaming HEVC mkv files from the Plex app.
Anytime I play one of my HEVC mkv files on the current 2022 model Roku Streaming Stick 4K I have an issue where the color looks very red especially on people's skin. I have two Roku Streaming Stick 4K players, exact same model and it's happening on both of them. This issue only happens when I play HEVC mkv files, when I play HEVC mp4 files those all look fine.
When I play the very same HEVC mkv files on my two 2017 model Roku Ultra players all the videos look fine, no high contrast or red color for the mkv files. Those files also look fine when I stream them on the Plex app for my two Fire TV devices. So there is definitely nothing wrong with the files themselves if other devices can display them correctly.
It's really irritating me that a Roku Ultra model from 6 years ago can stream these videos and they look perfectly normal but the current 2022 model Roku Streaming Stick 4K makes the videos look like garbage. How does this happen? Somebody screwed up and this needs to be addressed.
It can't be the latest Roku operating system that is the problem because I'm running the latest version on my Roku Streaming Stick 4K devices as well as my Roku Ultra devices. It must be a hardware related issue specific to the Streaming Sticks and it needs to be resolved.
Roku needs an update to fix this immediately because this is completely unacceptable! When are you going to do something about this Roku?