I think the problem on WebOS is the app is generic to every LG TV, and some TVs have support for different profiles. That is something Plex will somehow need to overcome. For example, the latest 10 range of OLEDs don’t seem to support profile 7 any more, whilst the older ones do. How does Plex differentiate between clients? Not sure.
There’s more on it here: Any plans for Dolby Vision support for LG webOS?
Part of the reason playback seems to be working reasonably well on Android based systems is due to Exoplayer, I don’t think Exoplayer is used for playback on Plex when used on WebOS but I’m not sure.
In the respect of testing, make sure that Plex isn’t transcoding or copying audio/video at all in the playback info. If it is, then everything will auto fall back to HDR10. Also don’t bother testing with the LG media player - it won’t have the updates to the latest MKV spec which includes Dolby Vision support. MP4 format can only contain profile 5 data, so I’m surprised if that works, that the profile 5 MKV doesn’t. That is certainly something Plex should be able to sort out. You should definitely report the problem to Plex.
