I stumbled across this in Ubuntu 20.04-based KDE neon, but in general it's sufficient to set GRUB_DISTRIBUTOR to a "wrong" value - the grub binary tries to load itself from a hardcoded path which ends in "ubuntu" and loading will fail and end in the grub shell.
This problem also affects
grub- efi-amd64- signed
on Focal Fossa if
GRUB_DISTRIBUTOR
is not set to "ubuntu" in /etc/default/grub.
I stumbled across this in Ubuntu 20.04-based KDE neon, but in general it's sufficient to set GRUB_DISTRIBUTOR to a "wrong" value - the grub binary tries to load itself from a hardcoded path which ends in "ubuntu" and loading will fail and end in the grub shell.
How to add Focal as an affected version?