yeah, that's not ideal, or possibly very bad UX, depending on how the update happens. (if the running stub resolver of resolved is not shut down, or at least an effort is made to fix resolve.conf during removal of the old package, then it might cover most of the users. as far as I understand the 11 -> 12 upgrade works without manual intervention, and this is an edge case, as systemd-resolved is not enabled by default on Debian 11.)