Description:
NetworkManager on Arch is being built with the flag --with-config-dns-rc-manager-default=resolvconf. Setting resolvconf as the default rc-manager makes the default systemd-resolved use detection by NetworkManager useless, since even when it detects that /etc/resolv.conf is a symbolic link to some of the systemd-resolved provided fallback files (like /run/systemd/resolve/stub-resolv.conf) and automatically pushes DNS changes to systemd-resolved, it will still overwrite /etc/resolv.conf using openresolv, which overwrites /run/systemd/resolve/stub-resolv.conf, something that should never happen.

Using rc-manager=symlink completely fixes this problem, since NetworkManager will understand that the user only wants NetworkManager to manage /etc/resolv.conf if it is a standard file. When that file is a symbolic link, NetworkManager will leave it alone and will not write over the symlinked file using resolvconf.

Please consider making, then, rc-manager default to symlink and not to resolvconf by building NetworkManager with the flag --with-config-dns-rc-manager-default=symlink, since it is probably a better default for cleanly handling all use cases (both when resolv.conf is a symlink and when it is not), and also makes the systemd-resolved detection actually useful.

Additional info:
* networkmanager 1.10.3dev+38+g78ef57197-1 (it is being built with --with-config-dns-rc-manager-default=resolvconf since 1.6.0-1).
* Excerpt from NetworkManager.conf(5): rc-manager/symlink: If /etc/resolv.conf is a regular file, NetworkManager will replace the file on update. If /etc/resolv.conf is instead a symlink, NetworkManager will leave it alone. Unless the symlink points to the internal file /run/NetworkManager/resolv.conf, in which case the symlink will be updated to emit an inotify notification. This allows the user to conveniently instruct NetworkManager not to manage /etc/resolv.conf by replacing it with a symlink.