To elaborate on the situation in Ubuntu focal, as there have been some improvements:
* grub-reboot(8) now works even when GRUB_DEFAULT != "saved", so that is out of the picture. I've edited the title of this bug accordingly.
* Both the man page and --help text for grub-set-default(8) indicate that GRUB_DEFAULT="saved" is required for the command to have the desired effect.
* grub-set-default(8) still gives no warning/error when GRUB_DEFAULT != "saved", however, leaving open the possibility of breaking scripts or other scenarios where it is assumed to work.
To elaborate on the situation in Ubuntu focal, as there have been some improvements:
* grub-reboot(8) now works even when GRUB_DEFAULT != "saved", so that is out of the picture. I've edited the title of this bug accordingly.
* Both the man page and --help text for grub-set-default(8) indicate that GRUB_DEFAULT= "saved" is required for the command to have the desired effect.
* grub-set-default(8) still gives no warning/error when GRUB_DEFAULT != "saved", however, leaving open the possibility of breaking scripts or other scenarios where it is assumed to work.