Updated, rebooted, and got stuck that the kernel image could not be found.
So it’s obvious that during the update, the 98-install-grub.hook hook didn’t run. Or more likely the same error was encountered as I did when I tried to run install-grub from a chroot environment.
Not the end of the world as I followed GRUB/Restore the GRUB Bootloader - Manjaro and was able to boot again. So I tried the install-grub script again, and all went well:
$ sudo install-grub
[...]
Grub will be installed on: EFI
Update UEFI Fallback file: /boot/efi/EFI/Manjaro/grubx64.efi
Rebooted and tested both kernels and it was good!