Comment 7 for bug 1918427

I wonder if this could be solved by changing the kernel's

Recommends: flash-kernel

to:

Recommends: grub-efi-arm64 | flash-kernel

That will prevent recommends from installing flash-kernel if grub-efi-arm64 is already installed. This also mirrors the existing Recommends selection for other architectures:

Package: linux-image-unsigned-5.10.0-12-generic
[...]
Recommends: grub-pc [amd64] | grub-efi-amd64 [amd64] | grub-efi-ia32 [amd64] | grub [amd64] | lilo [amd64] | flash-kernel [armhf arm64] | grub-ieee1275 [ppc64el], initramfs-tools | linux-initramfs-tool

Perhaps we can test this by uploading such a kernel to a PPA and doing a test curtin install on an EFI system to confirm that flash-kernel never gets installed. Even if it doesn't fix this issue, still seems like the correct thing to do in the kernel package.