Comment 2 for bug 1926528

For those using preseed and having the same problem, a workaround is to fix grub-installer in partman/early_command.

The following line in preseed.cfg will do the trick:

d-i partman/early_command string \
  sed -i "s/initial_proc_contents=\"$(ls $ROOT/proc)\".*/set +e ; & ; set -e/" /usr/bin/grub-installer ;