When you proceed to "Configure DHCP" and select all the required fields, the button "Configure DHCP" is greyed out (inactive).
The only way to make it "green" is to switch to "Relay to another VLAN" and back.
Checked on MAAS version 3.2.0 and current master (661748ba669df10ff68f0d0aa4d40e67977fe3b8)
When you proceed to "Configure DHCP" and select all the required fields, the button "Configure DHCP" is greyed out (inactive).
The only way to make it "green" is to switch to "Relay to another VLAN" and back.
Checked on MAAS version 3.2.0 and current master (661748ba669df1 0ff68f0d0aa4d40 e67977fe3b8)