Are you running Plex Media Server on the Synology NAS or on a Mac? You tagged the post “server-mac” but list a Synology DS1520+ as your server version.
You are running a very old version of Plex Media Server, released January 2020.
Consider updating to the current release, 1.24.5.5173. Among other things, it has the new Plex Movie agent, which is better at matching movies.
Plex Documentation → Your Media → Naming & Organizing Your Movie Files
Please show how you have the movies named and organized. Follow Plex recommendations for best results. Plex Dance the movies after any renaming / reorganizing. This removes incorrect metadata from the Plex database.
Example
/Movies <-- Movie library points here. Exact name unimportant.
.../Collateral (2004) <-- Subfolder for each movie
....../Collateral (2004).ext <-- ext = mkv/mp4/etc
.../Cast Away (2000)
....../Cast Away (2000).ext
