did the trick for me, but grub-efi-amd64-signed and shim-signed are persistently pending to finish the installation, and every time dpkg attempts to do it the efibootmgr changes mentioned above get wiped and they need to be re-run to be able to continue booting... not a very reliable way of getting a system booting reliably
grub-install --force --target=x86_64-efi /dev/md127 grubx64. efi grubx64. efi
grub-install --force --recheck /dev/md127
efibootmgr -c -D -L "Ubuntu (sdb)" -d /dev/sdb -p1 -l /EFI/ubuntu/
efibootmgr -c -D -L "Ubuntu (sda)" -d /dev/sda -p1 -l /EFI/ubuntu/
did the trick for me, but grub-efi- amd64-signed and shim-signed are persistently pending to finish the installation, and every time dpkg attempts to do it the efibootmgr changes mentioned above get wiped and they need to be re-run to be able to continue booting... not a very reliable way of getting a system booting reliably