What version of Plex Media Server?
If v1.20 or higher, which agent are you using, Plex Movie or Plex Movie (Legacy)?
Instead of just moving the files in/out of the library, do a full Plex Dance for both movies.
Add them back one at a time.
If Plex still combines them, then split them apart and perform a Fix Match on the one that is mis-identified.
Click on the three vertical dots in lower right corner of cover art to pull up the menu:
