Look at the Plex Media Info for the files. Are they Dolby Vision Profile 5?
Plex does not have a license from Dolby to tonemap DV to SDR.
Plex Desktop will direct play Dolby Vision Profile 5 files, but does not tonemap them to SDR.
Plex Media Server will not tonemap DV5 media when transcoding (the “Enable HDR tone mapping” setting applies only when Plex Media Server is transcoding the media).
There are several Dolby Vision profiles. Some include a fallback to HDR10. Dolby Vision Profile 5 does not.
When connected to a SDR display, Plex Desktop & Plex HTPC will direct play DV media. If the media has a HDR10 compatibility layer, Plex will play the compatibility layer and tonemap it to SDR. If there is no compatibility layer, Plex will play the media, but the colors will be off.
Plex Media Server tonemaps HDR10 to SDR only when transcoding HDR10 media. If a DV file contains a HDR10 compatibility layer, Plex Media Server will transcode and tonemap the compatibility layer to SDR (assuming PMS is configured to do so). If the DV file does not contain a compatibility layer, PMS will refuse to transcode the file and the client will display a “color space not supported” message.
