So, I run on a second laptop an "apt full-upgrade" (it could also be nala-upgrade or apt dist-upgrade), grub got its upgrade,
After that full-upgrade, I ran "grub-install /dev/sdb" (sdb is my boot disk) and afterwards "update-grub" (I don't know if this is necessary), rebooted, and I do not have any problems so far.
apt policy grub-common
grub-common:
Installed: 2.12~rc1-7
Candidate: 2.12~rc1-7