systemd-resolved explicitly (as in, it is documented behaviour) will not touch /etc/resolv.conf if it is a plain file.

It relies on /etc/resolv.conf being a symbolic link to it's generated version in /run/systemd/resolve/resolv.conf to do its thing. If it isn't linked, it assumes that is intentional and leaves your resolv.conf alone.

We'll use inotifywait then.