Full title is Glass Onion: A Knives Out Mystery. Source: https://twitter.com/netflix/status/1536379398906974209

“A Knives Out Mystery” is the subtitle/tagline, like how “A New England Folktale” is the subtitle to The Witch (2015) (listed on this site as simply, The Witch), and Rían Johnson’s tweet refers to it simply as “Glass Onion” (Source: https://twitter.com/rianjohnson/status/1536379230526722049?s=21&t=QgiggKNdH31VejPdlYKheQ) in the text, so I think they should just rename it to Glass Onion, and leave the Full Title+Subtitle as an alt title.

The Netflix page has been updated and shows Glass Onion: A Knives Out Mystery as the title. Glass Onion can be added as an alternative title.

@Yitzinig said:

“A Knives Out Mystery” is the subtitle/tagline, like how “A New England Folktale” is the subtitle to The Witch (2015) (listed on this site as simply, The Witch), and Rían Johnson’s tweet refers to it simply as “Glass Onion” (Source: https://twitter.com/rianjohnson/status/1536379230526722049?s=21&t=QgiggKNdH31VejPdlYKheQ) in the text, so I think they should just rename it to Glass Onion, and leave the Full Title+Subtitle as an alt title.

The official "title announcement" has the subtitle as part of its title. https://www.youtube.com/watch?v=ieEF6Mm4Y0w

“A New England Folktale” was NEVER meant to be a subtitle of the Witch. That was a tagline.