It appears that Ubuntu 18.04 switched network stacks to Netplan.  This causes some strangeness when trying to set up DHCP reservations on Windows DHCP servers, as the physical address Netplan sends to Windows is not the MAC address of the interface.

From https://netplan.io/examples#windows-dhcp-server

Edit the config file under /etc/netplan/ to look like this:


network:
version: 2
ethernets:
enp3s0:
dhcp4: yes
dhcp-identifier: mac

Afterwards, run sudo netplan apply