Have the same bug is in Ubuntu 20.04.5 and isc-dhcp-server 4.4.1-2.1ubuntu5.20.04.4
It's very sad that bug is still haven't been fixed since 2015(!) year.
For myself I solve it with rc.local: sleep 5 /usr/bin/systemctl restart isc-dhcp-server
Have the same bug is in Ubuntu 20.04.5 and isc-dhcp-server 4.4.1-2. 1ubuntu5. 20.04.4
It's very sad that bug is still haven't been fixed since 2015(!) year.
For myself I solve it with rc.local:
sleep 5
/usr/bin/systemctl restart isc-dhcp-server