Raspberry pi Playback failed due to a fatal player error.

[TheDreadPirate]

Plex uses their own playback engine while Jellyfin relies on the playback engine that Google providers (exoplayer). And there are known issues with exoplayer not properly handling HDR content. Or not falling back to compatible modes. If this is a Dolby Vision profile 8 video, Plex is probably falling back to SDR since that profile supports falling back to SDR.

But exoplayer does not gracefully handle fallback in a lot of cases.

It is a known pain point and there are efforts to workaround this.