It sounds like you are having trouble with tonemapping HDR to SDR on the fly. This is a non-trivial task, but not impossible. Both mpv and ffmpeg (which plex and jellyfin use) are capable of this. If you install mpv, it will by default do the tonemapping, you can enable/disable this or force use of a particular algorithm if you like.
To answer your question: Plex has been pretty shitty for years now, and it’s only getting worse. They just don’t care for their user base.
ETA: Jellyfin also already does what you want, I think?
It sounds like you are having trouble with tonemapping HDR to SDR on the fly. This is a non-trivial task, but not impossible. Both mpv and ffmpeg (which plex and jellyfin use) are capable of this. If you install mpv, it will by default do the tonemapping, you can enable/disable this or force use of a particular algorithm if you like.
To answer your question: Plex has been pretty shitty for years now, and it’s only getting worse. They just don’t care for their user base.
ETA: Jellyfin also already does what you want, I think?