There are two parts to the Plex LG app, a loader version, that is installed on the TV, and the full client that is downloaded when you launch the app.
The loader, which is 5.1.2, is not the full app. It exists only to download the full app when launched.
To see which version is downloaded, launch the app, then go to Settings → About → Client Version.
You can also check Authorized Devices.
The current version is 5.43.2: Announcement
