When selecting a movie in either the Windows Plex app or the Plex web browser page the Add to Watchlist button/icon is missing from under the movie poster. It was there up until a few weeks ago. The only way I found to add a movie to the default Watchlist is to play a movie trailer and there I can see the option in the player window to add the movie to the Watchlist.
I am using Plex Movie as the scanner/agent, I have re-scanned and done the Plex dance.
The agent was already set to Plex Movie without the word Legacy.
None of my libraries have the checkmark for ‘include adult content’ set, it was already blank.
Without changing anything the problem appears to have solved itself. The software did receive an update a little while ago without any immediate change to the problem but some time has passed since the update and so maybe something magical happened over time with the software. I shall see how it goes and update here if the problem comes back.