Automating 19.04 Installation

I've been trying to automate my installation of 19.04 using pre-seeding files. I'm booting directly from installation media copied to a USB stick.

So far, nothing I've tried has worked. Subiquity loads as normal and presents the user interface to manually install.

This is my grub.cfg

if loadfont /boot/grub/font.pf2 ; then
    set gfxmode=auto
    insmod efi_gop
    insmod efi_uga
    insmod gfxterm
    terminal_output gfxterm
fi

set menu_color_normal=white/black
set menu_color_highlight=black/light-gray

set timeout=1
menuentry "Install Ubuntu Server" {
    set gfxpayload=keep
    linux   /casper/vmlinuz  file=/cdrom/preseed/magicmirror.seed boot=casper quiet splash ---
    initrd  /casper/initrd
}

My pre-seeding file is from here: https://github.com/core-process/linux-unattended-installation/blob/master/ubuntu/18.04/custom/preseed.cfg

I'm not expecting it to work unchanged, but I was expecting it to error or give some indication that the pre-seeding file had been loaded.

I've also tried loading a Kickstarter but I got the same result.

Am I loading the pre-seeding file wrong, or has automated installation changed in 19.04?