mkfs creates ext4 volumes with “Filesystem features” that include ‘64bit’.
The bootloader does not really like this and fails to boot as a result.
replacing a line in flash.sh from:
mkfs -t $4 "${loop_dev}" > /dev/null 2>&1;
to:
mkfs -t $4 -O ^64bit,^metadata_csum "${loop_dev}" > /dev/null 2>&1;
fixes the issue for me