Out of curiosity, can you try downgrading to see if tone mapping starts working properly?
I have what I think is the same problem you’re reporting - on both Fedora 34 and 35 - and if I downgrade to plexmediaserver-1.22.0.4163-d8c4875dd (found here: GitHub - axlecrusher/plex_download_urls: Archive of Plex mediaserver download URLs), transcoding / tonemapping is done in hardware.
