Grub remembers where to reinstall.
To see what drive grub2 uses see this line - grub-pc/install_devices, check what it is now:
sudo debconf-show grub-pc
to get grub2 to remember where to reinstall on updates, grub-pc is for BIOS and grub-efi is for UEFI, if you unselect everything then the command above should change to a blank or grub will not reinstall.
sudo dpkg-reconfigure grub-pc
sudo dpkg-reconfigure grub-efi-amd64
Enter thru first pages,spacebar to choose/unchoose drive, enter to accept, do not choose partitions
You can also just change to install grub to a partition. We suggest never to install to a partition as it does not work well, but if really booting with another copy of grub that is not really an issue.
How to: Create a Customized GRUB2 Screen that is Maintenance Free.- Cavsfan