As you allow the sand box to access the host file system and access the file over the file dialog you get a very different path than the expected. actual: Access file /home/username/testproject/testproject.pro => /run/user/1001/doc/3b985a78/testproject.pro expected: Access file /home/username/testproject/testproject.pro => /home/username/testproject/testproject.pro I think it is because the file dialog portal is not testing if the file access is permitted and sends always an opaque file handle.
I discussed this with Flatpak developers and opened a bug here: https://github.com/flatpak/xdg-desktop-portal/issues/141
Good, I was not sure it belonged to kde or flatpak in general. Thank you.
I believe this is no longer a problem.