The issue seems to be related to GRUB. If I upgrade my working-in-SB-disco VM to focal's grub-efi-arm64-signed, it now fails. If I downgrade my *not*-working-in-SB-focal VM to disco's grub-efi-arm64-signed, it now boots in SB mode.
The issue seems to be related to GRUB. If I upgrade my working-in-SB-disco VM to focal's grub-efi- arm64-signed, it now fails. If I downgrade my *not*-working- in-SB-focal VM to disco's grub-efi- arm64-signed, it now boots in SB mode.