When you install the Fall Creator Update insider preview and try to direct play a hevc video in Plex, you'll notice that the screen stays black and you hear only the sound.
The reason is that Microsoft has moved the hevc playback capability into an extension named "Hevc video extension". It is not visible if you do a search directly in the Windows Store. You need to try to play an hevc file in the "Movie And TV" app, then it will give you the option to download the extension. Once it is installed, the Plex app will direct play Hevc files without problem.
This procedure was tested on build 16257 of the insider preview.