installer that works for both BIOS and EFI

I have an installer that works correctly when booting EFI but not BIOS. I've seen several examples that show stuff like this:

from https://raw.githubusercontent.com/core-process/linux-unattended-installation/master/ubuntu/20.04/custom/preseed.cfg:

---------------------------------

...

### GPT
d-i partman-basicfilesystems/choose_label string gpt
d-i partman-basicfilesystems/default_label string gpt
d-i partman-partitioning/choose_label string gpt
d-i partman-partitioning/default_label string gpt
d-i partman/choose_label string gpt
d-i partman/default_label string gpt

### EFI
d-i partman-efi/non_efi_system boolean true

### Grub
d-i grub-installer/only_debian boolean true
d-i grub-installer/with_other_os boolean true

### Disk layout

# Keep 25% free space
d-i partman-auto/expert_recipe string \
boot-root :: \
1 1 1 free \
$bios_boot{ } \
method{ biosgrub } \
. \
256 256 256 fat32 \
$primary{ } \
$iflabel{ gpt } \
$reusemethod{ } \
method{ efi } format{ } \
mountpoint{ /boot/efi } \

...

---------------------------------

My config (https://github.com/idaholab/Malcolm/blob/master/sensor-iso/config/includes.binary/install/preseed_base.cfg and https://github.com/idaholab/Malcolm/blob/master/sensor-iso/config/includes.binary/install/preseed_vmware.cfg) looks similar, but the installer hangs at the end at a blank screen with a blinking cursor.

I'm able to activate another terminal and I see repeated messages with "process '/sbin/debian-installer' (pid 12345) exited. Scheduling for restart." It works fine with EFI installation, however.

I've obviously done something wrong, either in the preseed or somewhere else. Any tips on how I can narrow it down, or what I might be doing wrong?

-SG

--
Seth Grover

Be kind to all of your neighbors
Because they're just like you.
And you're nothing special
Unless they are too.