Update fails when it contains kept back packages

[ghost]

AppCenter is working regularly with all other updates except:

“4 components with updates:
• grub-common GRand Unified Bootloader (common files) Version: 2.02-2ubuntu8.23+elementary27ubuntu5.1.7.1
• grub-pc-bin GRand Unified Bootloader, version 2 (PC/BIOS binaries) Version: 2.02-2ubuntu8.23+elementary27ubuntu5.1.7.1
• grub2-common GRand Unified Bootloader (common files for version 2) Version: 2.02-2ubuntu8.23+elementary27~ubuntu5.1.7.1”

on prompting the update button an empty cursor appears briefly indicating “loading cache” and then suddenly disappears with no effects.
Tried sudo apt update, sudo apt-get update, and sudo apt upgrade, sudo apt-get upgrade resulting in: “The following packages have been kept back:
grub-common grub-efi-amd64-bin grub-efi-amd64-signed grub-pc grub-pc-bin
grub2-common shim shim-signed
0 upgraded, 0 newly installed, 0 to remove and 8 not upgraded.”

elementary OS 5.1.7 Hera
Built on Ubuntu 18.04.6 LTS
Linux 5.4.0-89-generic
GTK 3.22.30
MacBook A1181
Dual-Core Intel® Core™2 Duo CPU T8300 @ 2.40GHz
Intel Corporation Mobile GM965/GL960 Integrated Gr..
2,0 GB memory

[ghost]

After trying to install the single packages one by one it would appear that this involves the removal (temporary?) of an essential package (shim/shim-signed), and I suppose the system refuses to do so. Apparently a bug (?)

[immutable-pro]

Packages that are kept back shouldn't show as updates in App Center, in my opinion, as they aren't actionable and AppCenter then occupies a slot in Plank with a count badge, but again, no action is possible.

[immutable-pro]

This duplicates: #116, as it is the same root cause.