Not sure if this will help. i'm on unraid 7's beta, I'm running plex with the plexpass build. I was having trouble with my files being hardware transcoded with my Arc380 card. I was able to pass thru the card with extra parameter
--device=/dev/dri:/dev/dri
and at that point anything that was not 4k worked great, but nothing 4k would.
turned on the console in settings and started playback. I saw an error concerning the EAC codecs... so did some googling and found this
https://www.reddit.com/r/PleX/comments/13mxbfd/eac3_transcode_errors_eae_timeout/
I opened my app data folder, navigated to here:
appdata\plex\Library\Application Support\Plex Media Server\Codecs
deleted the folders, because the thread told me Plex would redownload them, restarted plex, and everything is working fantastically. hw encoding, tone mapping, all of it.
I hope this helps someone.
I'm using the linuxserver plex container, in case that makes a difference. But this thread helped me troubleshoot to get as far as I did, so I'm sharing my results here.