*nix noob here. I moved a Plex server from Windows and everything is working flawlessly except HDR. The client side should be good (NVIDIA Shield and LG OLED TV that both rendered HDR fine under Windows) and I know my server hardware (10th gen i7 NUC) is capable so I’m assuming I’m just missing some packages and/or config. This is a fully-updated EndeavourOS installation but I think I remember having the same issue under raw Arch.
Jellyfin only l does HDR to SDR transcoding (unless I’m out of date). I had to turn off transcoding to ensure the HDR content was handled by the receiving hardware instead. I believe HDR to HDR transcoding is coming to Jellyfin, as it was held up by their version of ffmpeg that now has the needed feature merged, but I’m yet to see it available in Jellyfin itself.