While testing the passwordless-gdm feature, I noticed that the QR code shown by GDM is the regular verification_uri.
It would be nice if verification_uri_complete would be used for the QR code, if provided.
On the SSSD side, most of the things to do this seem to be in place already.
I drafted a PR for this here.
The gnome-shell PR for the passwordless-gdm feature is still in review, so I would ask there next, unless there is a better place to ask for this change.