If only IPv6 is configured for a machine, then we should not be relying on ANY IPv4 addresses. log_host would be part of "ANY", as seen below:
BOOT_IMAGE=ubuntu/amd64/hwe-x/xenial/daily/boot-kernel nomodeset iscsi_target_name=iqn.2004-05.com.ubuntu:maas:ephemeral-ubuntu-amd64-hwe-x-xenial-daily iscsi_target_ip=2001:67c:1562:8010::2:1 iscsi_target_port=3260 iscsi_initiator=kearns ip=::::kearns:BOOTIF ro root=/dev/disk/by-path/ip-2001:67c:1562:8010::2:1:3260-iscsi-iqn.2004-05.com.ubuntu:maas:ephemeral-ubuntu-amd64-hwe-x-xenial-daily-lun-1 overlayroot=tmpfs cloud-config-url=http://maas-boot-vm-xenial.dt-maas/MAAS/metadata/latest/by-id/8w4gkk/?op=get_preseed log_host=10.246.0.5 log_port=514 --- console=ttyS1 BOOTIF=01-38:63:bb:43:b8:bc