The original primary issue (that pid files were not being written at all) was resolved in isc-dhcp 4.3.3-5ubuntu9. However both isc-dhcp-server & isc-dhcp-server6 systemd units continue to use the same RuntimeDirectory, so that when either service is restarted (eg to reconfigure), the pid file of the other service is lost.
The original primary issue (that pid files were not being written at all) was resolved in isc-dhcp 4.3.3-5ubuntu9. However both isc-dhcp-server & isc-dhcp-server6 systemd units continue to use the same RuntimeDirectory, so that when either service is restarted (eg to reconfigure), the pid file of the other service is lost.