Right, and (many years ago) they added automatic "failback", as well as the "failover" behaviour that systemd-resolved has. So this isn't a case of systemd-res

Right, and (many years ago) they added automatic "failback", as well as the "failover" behaviour that systemd-resolved has. So this isn't a case of systemd-resolved deciding to arbitrarily do things in a new and broken way; rather it's systemd-resolved still being rather immature (but for some reason already ending up as the default on lots of people's systems).