mp4 files are notorious for containing embedded metadata. And these metadata are more often than not incorrect.
Your best course of action is to remove those metadata completely.
You can use Windows’ file explorer to do it or a special tagger software, like mp3tag.
Afterwards, perform the Plex Dance with those movies.
You’ll have to delete the collections, which were made from the embedded meta tags, manually. https://support.plex.tv/articles/201273953-collections/
