Just making sure you know there’s two places to adjust plex settings. One is the player you are using, which I’m guessing is Plex Media Player on Windows. Other settings are on your server, whether or not it’s on the same machine. That is accessible at plex.tv/web on your local network. I’ve found that plex can be a little stupid with what it thinks is local or remote and therefore will try to transcode when it shouldn’t. There may be some kind of inconsistency between the two?
Hope you get it figured out. I’ve been a plex user for many years after using xbmc/kodi. While kodi is very powerful, plex is dirt simple for my entire family to use, and therefore why I wholeheartedly recommend it.