Movie and sequel match correctly but won't both show at the same time

Plex Media Server Metadata & Adding Files
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Considering that these are mp4 files – did you check for embedded meta tags?

  1. If you have enabled “Prefer local meta tags” in the properties of your movie library, consider turning it off. It usually isn’t advisable enabling it unless you’re exerting tight control over all embeded meta tags in your files.
  2. Both movies are R-rated. They will only match if “Include adult content” has been enabled in the properties of the library.
  3. Perform the Plex Dance with both movies. Add them back individually.