I guess Plex is mismatching the videos as duplicates of one of them. If you click 
and go to the Info tab… does it list all 8 movies?
Side note: if you experience this with a different movie and don’t know into which movie the different files got merged you can also filter your library for Duplicates to identify those.
In order to fix this you have 2 options:
- [Quick Win] Click on
⋮>Split Apart(from the context menu of that movie) and subsequently manually match the resulting individual items. - [Sustainable] Rename your files to reflect the correct movie title / release year and add those files to your library again (probably you’ll need to do a Plex Dance to complete this to undo the incorrect match – this will be more work it’ll save you future effort if you ever have to rebuild your library
Example:
Harry Potter and the Prisoner of Azkaban (2004),Harry Potter and the Goblet of Fire (2005)…
