flatpak permission from manifest does not grant permission to xdg-documents

When I built a flatpak, I put this in my manifest yml:

finish-args:
  - --filesystem=xdg-desktop
  - --filesystem=xdg-documents
  - --socket=x11
  - --share=ipc

Then I built the app and installed it.

flatpak-builder --user --install build --force-clean com.example.helloworld.yml

When I run the program and use a file picker dialog (using Tkinter in python3), I see only three directories in my "home" directory: .local, .var, and Desktop. **Why is ~/Documents not in the list, when ~/Desktop is?** When I give a specific override to the path, it works.

$ flatpak override --user com.example.helloworld --filesystem="${XDG_DOCUMENTS_DIR}"

Then the file dialog shows me my Documents directory. I had to hard-code a filesystem path, which is the defined XDG_DOCUMENTS_DIR (and is the default one even if that is not defined specifically).