Plex won't "forget" a movie after it is deleted (so I can re-add it with new chapter stop names)

The movie is Aliens (1986). I had a directory (Aliens (1986)) inside my Media/Movies directory with two files in it: ‘Aliens SE (1986).m4v’ and ‘1986 Theatrical Version-other.m4v’. I deleted everything (files and directory), and even emptied the recycle bin on the drive before beginning the procedure. After finishing the Plex dance, I re-added everything as before.