is literally the first line of the postrm for the purge action.
There doesn't appear to be a way to solve this because we do need to remove the package and grub-pc and grub-efi-amd64 own this.
However this is a low priority issue in any case, as you need to forcefully switch the metapackage on the system which is both unnecessary and hard to do.
Yes, that seems the expected behavior given that
rm -f /etc/default/grub
is literally the first line of the postrm for the purge action.
There doesn't appear to be a way to solve this because we do need to remove the package and grub-pc and grub-efi-amd64 own this.
However this is a low priority issue in any case, as you need to forcefully switch the metapackage on the system which is both unnecessary and hard to do.