Correction to my earlier comment: it is the update-initramfs trigger during the linux-signed-image-$abi install that handles making the initramfs, update-grub2 only has to find it. :)
Correction to my earlier comment: it is the update-initramfs trigger during the linux-signed- image-$ abi install that handles making the initramfs, update-grub2 only has to find it. :)