Plex Transcoding Issue - Audio and Black Screens

I figured I would post this since I haven't found it too readily available.  When dealing with Plex transcoding, I recently ran into an issue where:

• Subtitles would not be properly rendered.  Result was a black screen with audio and nothing else.
• Transcoding from a 4k MKV would not work (usually to a lower resolution), again resulting in a black screen with audio and nothing else.
• Some Plex apps (Rokus in particular) were only displaying black screens with audio for some movies.
• For any of the above, I also noted that the GPU was never being utilized.

Possible Solution:

• Turn off Enable HDR tone mapping in Plex WebUI.
  • Not really sure why this solved the issue but every one of the above problems was fixed.