Is your media storage outside of the Plex server device?
If so, keep this Disabled:
Settings - Server - Library - “Empty trash automatically after every scan”
Is it just one movie? And does this movie always stay as “the most recently added”? Meaning other movies you add don’t bump this particular movie down the recently added list…
If this is the case then I suspect the movie was either added when your system date was wrong(giving this movie a timestamp way in the future) and/or the database timestamp entry got corrupt when the movie was first added.
In either case I bet you can solve the issue via Plex Dance
Check the modified date timestamp of the file in question in your file manager. If this is Windows, File Explorer will show blank if the time range is outside of 1970-2101.
If that’s the issue, depending on if its Windows or Linux you can use Powershell or Touch to modify the timestamp to the value you want. Then when you add new files to plex you should see it move down as normal.