Comment 8 for bug 1838525

Was able to reproduce, cdebconf shows:

----

Name: grub-installer/bootdev
Template: grub-installer/bootdev
Value: /dev/mapper
Owners: grub-installer

Name: grub-installer/grub-install-failed
Template: grub-installer/grub-install-failed
Owners: grub-installer
Variables:
 BOOTDEV = /dev/mapper

Name: grub-installer/progress/step_install_loader
Template: grub-installer/progress/step_install_loader
Owners: grub-installer
Variables:
 BOOTDEV = /dev/mapper

Name: nobootloader/confirmation_common
Template: nobootloader/confirmation_common
Owners: nobootloader
Variables:
 ROOT = root=/dev/mapper/vgubuntu-root
 BOOT = /dev/mapper/vgubuntu-root
 KERNEL = /boot/vmlinuz

----

this debconf template comes from grub-installer package, using BOOTDEV variable (which is wrongly set to /dev/mapper) from:

grub-installer file:

db_progress STEP 1
db_subst grub-installer/progress/step_install_loader BOOTDEV "$bootdev"
db_progress INFO grub-installer/progress/step_install_loader

I'll do the same logic - as this shell script - in installation console to check why "default_bootdev" and "bootdev" are being wrongly set.