The mpv.conf is not in the Plex HTPC install folder.
Quote:
This file is located at these locations (it doesn’t exist by default so create it if you don’t have one):
Windows: %LocalAppData%\PlexHTPC
MacOS: ~/Library/Application Support/Plex HTPC/
Linux: ~/snap/plex-htpc/common
Note: Sometimes when creating this file, you can accidentally create it as mpv.conf.txt and the .txt will be hidden from you. Be sure you create it with the correct name or it will be ignored.