Same on MacbookPro Late 2013, Big Sur, Xcode 13.2.1

Found a way to set the values: Add Document Types (Like you did), then go to the project's info file and add one value (like: "Document Extensions") to the . The when I close and reopen my project the "additional document type properties" are available in Document Types. I do not know how to upload an image to the dev. forum, so I can't show how. (How did you post the image?)