That’s just added to my confusion 
Plex settings says “Client Version 5.64.1 - Platform Version 5”
Authorised devices says the new version. So that seems fine, guess plex settings is just showing the loader version
However, I do wonder if you found the issue here Samsung TV app Black Screen - #12 by auto98uk (investigating which is what made me notice the versions) - authorised devices was also showing the old version as being used AFTER the last entry of the new version on the same devices - makes me wonder if something on the plex end was serving the wrong version of the app.
