It took a while to grep the vol_id culprit. It's probably this:
/usr/share/initramfs-tools/scripts/functions: elif [ "$FSTYPE" = "unknown" ] && [ -x /lib/udev/vol_id ]; then
/usr/share/initramfs-tools/scripts/functions: FSTYPE=$(/lib/udev/vol_id -t "${FS}" 2> /dev/null)