This is "by design"[0], though you should have seen it crop up in 17.04 when it became the default. I was bitten by it too and disabled systemd-resolved.
0: https://github.com/systemd/systemd/issues/6224#issuecomment-...
0: https://github.com/systemd/systemd/issues/6224#issuecomment-...
Of course, setting the following in /etc/systemd/resolved.conf did no good:
LLMNR=no
...because apparently systemd-resolved just ignores its configuration. (I did restart the service; it made no difference.) I re-commented that setting since it does nothing. In order to get this working, I instead had to change the symlink /etc/resolv.conf which previously pointed to /run/resolvconf/resolv.conf to instead point to /run/systemd/resolve/resolv.conf, another dynamic file which is also maintained by systemd-resolved.So now I have systemd-resolved handling the info from the DHCP ack, but it isn't handling DNS. This works, but good grief.