Comment # 1 on bug 1136753 from Vinzenz Vietzke

That has nothing to do with Xfce but with flatpak.

Looking at flatpak spec file I see xdg-desktop-portal as dependency:
https://build.opensuse.org/package/view_file/openSUSE:Factory/flatpak/flatpak.spec

This should imho be enough, right?