There is no such thing as a local watchlist in Plex. All Plex watchlists live on plex.tv.
So the only explanation I have is that you are actually using a different plex user account on that TV.
You have a Plex Home set up, so switching users is easily done. Maybe you switched accounts (or someone else did while you weren’t looking)?
I am definitely on the same account as the same user.
It also definitely a ‘local’ account! When I do switch users on the TV the same watchlist shows up for all of us (on plex.tv they do not share, are all personalised)
i.e. if you clean out the field “List of IP addresses and networks that are allowed without auth” in your server’s preferences and restart the server, can you still use this TV to access its media content?
If not, the TV is too old and cannot fill up the watchlist at all.
I wonder if the “local” watchlist items on that TV will survive a cold boot of the TV.
I just heard that there was a bug that has been fixed recently, where some data of the app weren’t reloaded properly after a user switching. Maybe that is what you are seeing?
Can you determine the exact version number of your player app?