subiquity 21.08.2 preseeded install fails

I don't have good logs to attach yet, but I have a reproducer, so this should still be useful.

Steps:

1. Download the latest impish-live-server-amd64 image.
   Currently this is:

bsdtar -x -f impish-live-server-amd64.iso -O \./.disk/info
Ubuntu-Server 21.10 "Impish Indri" - Daily amd64 (20211007

2. qemu-img create -f qcow2 disk1.qcow2 10G
3. truncate -s 16M answers.img
4. mkfs.ext2 -U 00c629d6-06ab-4dfd-b21e-c3186f34105d answers.img
5. mkdir tmpmnt
6. sudo mount answers.img tmpmnt
7. Copy the answers.yaml file below to tmpmnt/
8. sudo umount tmpmnt
9. Install

kvm -m 2048 -boot d -cdrom ./impish-live-server-amd64.iso -drive file=disk1.qcow2,if=virtio -drive file=answers.img,if=virtio

Several failures should be visible during the subiquity run. It will eventually stop with a black screen. It is still possible to switch to a tty for debugging. Interestingly doing (what I believe are) the same steps manually works.

Best case scenario: I preseeded the source screen badly.
Note: from the preseed below I intentionally left out the reboot answer.

--- answers.yaml ---

Welcome:
  lang: en_US
Refresh:
  update: yes
Source:
  source: ubuntu-server
Keyboard:
  layout: us
Installpath:
  path: ubuntu
Zdev:
  accept-default: yes
Network:
  accept-default: yes
Proxy:
  proxy: ""
Mirror:
  country-code: us
Filesystem:
  guided: yes
  guided-index: 0
# Utah fills out the Identity section by itself
SnapList:
  snaps: {}