Hi!
Just did a d-i bare-metal test run; installing to another disk, with the
obvious requirement of not damaging the primary system.  Thus, I explicitly
marked all relevant partitions (EFI, sys, and swap) as "do not use".

Yet the newly installed system overwrote the ESP anyway.  It also did so in
a way that neither the old nor new system could be booted (no entries for
any of the existing two systems were created, and I did not succeed booting
manually).

Disks present in the system:
* NVME-SSD:
  [all "do not use"] ESP (fat), sys (btrfs), swap
* 4x NVME-Optane:
  MD RAID0 <- new system (d-i test) was installed here
* HDD:
  * another old system (ext4) -- x32, BIOS-boot
  * boot partition for the d-i test run
  * data partition (btrfs)

It can be argued that the setup above may be a bit overcomplicated (thus
the installer being confused might be understandable).
But I insist that disregarding the explicit "do not use" and scribbling
over anyway is a severe bug.

Control: reassign -1 grub-installer

This seems as though it can only reasonably be fixed in grub-installer,
since it's what explicitly calls grub-install here.
grub-efi-amd64.postinst won't independently call grub-install in this
setup (since /boot/efi won't be mounted in the installed system, so the
-d "/boot/efi/EFI/$bootloader_id" test will fail).  Reassigning.

When installing in grub-efi* mode, I think it would probably be
reasonable for grub-installer to only call grub-install if
$ROOT/boot/efi is a mountpoint.

How was the md RAID0 array partitioned ? The provided information does
not show any partitions on /dev/md127. Anyway, grub-install cannot
create an EFI boot entry when /boot/efi is on md RAID or any other
abstraction layer, it must be a plain partition on a physical disk so
that the UEFI firmware can read it.

grub-installer does not select nor format the EFI partition. It just
runs grub-install, and grub-install writes into whatever was mounted on
/target/boot/efi by partman-efi. partman-efi and partman-auto-lvm have
glitches which may cancel the user's choice to not use an EFI partition.
I am working on fixes.
Before committing the partitioning changes, partman-efi warns the user
if no EFI partition is found and offers to resume partitioning.