I seem to have the same bug under Linux Mint 20.1 on a late 2008 MacBook.
After some upgrade a few months ago, I've got a [problem with shim-signed][1], where I was getting "Failed to set MokListRT" error and no GRUB menu. After purging shim-signed, the error messages were gone, but the GRUB menu did not come back.
I tried the solution linked to from comment #23, and it worked, but
(1) the menu does not look the same as before,
(2) there seem to be somewhat unusual timeouts before the menu is shown and after a boot option is selected, but I may be wrong.
I seem to have the same bug under Linux Mint 20.1 on a late 2008 MacBook.
After some upgrade a few months ago, I've got a [problem with shim-signed][1], where I was getting "Failed to set MokListRT" error and no GRUB menu. After purging shim-signed, the error messages were gone, but the GRUB menu did not come back.
I tried the solution linked to from comment #23, and it worked, but
(1) the menu does not look the same as before,
(2) there seem to be somewhat unusual timeouts before the menu is shown and after a boot option is selected, but I may be wrong.
The workaround suggested in comment #23 is to set
GRUB_ TERMINAL= console
in /etc/default/grub.
[1]: https:/ /bugs.launchpad .net/ubuntu/ +source/ grub2/+ bug/1867092