system installation – ubuntu autoinstall

I’m building my own custom Ubuntu with the subiquity and the following reference:

autoinstall-reference

My issue is with the storage part. Apparently it’s possible to automatically select the smallest disk by adding size: smallest to the storage key in the schema. This does not work for me at the moment. Here’s my storage definition:

  storage:
    config:
      - {ptable: gpt, preserve: false, name: '', grub_device: false,
         type: disk, size: smallest, id: disk-vda}
      - {device: disk-vda, size: 536870912, wipe: superblock, flag: boot, number: 1,
         preserve: false, grub_device: true, type: partition, id: partition-0}
      - {fstype: fat32, volume: partition-0, preserve: false, type: format, id: format-0}
      - {device: disk-vda, size: -1, wipe: superblock, flag: '', number: 2,
         preserve: false, type: partition, id: partition-1}
      - {fstype: ext4, volume: partition-1, preserve: false, type: format, id: format-1}
      - {device: format-1, path: /, type: mount, id: mount-1}
      - {device: format-0, path: /boot/efi, type: mount, id: mount-0}

Did anyone get this to work? Am I doing anything wrong? What’s noticeable is that the kv-pair disk:smallest is missing from the generated autoinstall-user-data file in /var/log/installer