Comment 64 for bug 1186662

eproust, since yours was a fresh install the cause of your problem is unlikely to be the same. But you may want to install the debsums package and check whether any of the files on your system differ from the ones in the package:

$ sudo apt install debsums
$ debsums -e isc-dhcp-server

You may also want to check that your systemd unit is the expected one, by running the following command and attaching the resulting file:

systemctl show isc-dhcp-server > isc-dhcp-server-systemctl.txt