One of the things that irritates me about fancy new service management systems like systemd is that unless you get everything exactly right, you can end up with things interfering with specific configuration files – specifically /etc/resolv.conf.
Now as a DNS administrator, I have a certain fondness for manually controlling /etc/resolv.conf and it does actually come in useful for making temporary changes to test specific DNS servers and the like. The trouble comes when something else wants to control that file.
The ideal fix for this conflict is to have things like systemd control a separate file such as /etc/system/resolv.conf.systemd, and for /etc/resolv.conf be installed as a symbolic link pointing at the real file.
But back in the real world, if you do disable systemd-resolver which can be done with:
systemctl disable systemd-resolved.service;
systemctl stop systemd-resolved.service
Then you may also want to make the file immutable:
chattr +i /etc/resolv.conf. On at least one server, systemd merrily re-created /etc/resolv.conf as a symbolic link to an empty file despite systemd-resolved being disabled.