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?)
Kaas Coder
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Same on MacbookPro Late 2013, Big Sur, Xcode 13.2.1