Found on a related PLEX forum for LG TV’s:
markus101Plex Employee
Thanks for the ping.
This came up in our error reporting and has already been fixed, but not yet released. It’ll be in the next release which should be going out early next week.
I believe the only way to reset the settings for webOS applications is to do a full factory reset, uninstalling and installing the Plex app again will not clear the settings from storage.