The installer was run in a live system booted from a read-only USB stick that has an EFI system partition. This partition was mounted at /boot/efi/ by the installer (despite never being requested, and the system not using UEFI). This particular behavior has been reported in other bugs, e.g. bug #1983384, bug #1970995.
If /boot/efi is unmounted, and the directory removed during installation (prior to the grub-install step), then the installation completes successfully.
A couple further remarks:
The installer was run in a live system booted from a read-only USB stick that has an EFI system partition. This partition was mounted at /boot/efi/ by the installer (despite never being requested, and the system not using UEFI). This particular behavior has been reported in other bugs, e.g. bug #1983384, bug #1970995.
If /boot/efi is unmounted, and the directory removed during installation (prior to the grub-install step), then the installation completes successfully.