I’m running Plex on a Synology using all latest software versions.
When using the web interface to create and maintain my playlists, I just tried to add some newly added movies to existing playlists.
But only one of five playlists show up when selecting the “Add to playlist…” menu option which appears after clicking the “…” of a movie.
Smart Playlists are not appearing in this selection, because they only follow the filter rules which were used to create them and are therefore updating automatically.
You can tell a smart playlist by the ‘cog wheel’ icon on top of its poster.