For your current work around to persist, you should also do:
$ sudo dpkg-divert /usr/lib/shim/shimx64.efi.signed $ sudo cp /usr/lib/grub/x86_64-efi-signed/grubx64.efi.signed /usr/lib/shim/shimx64.efi.signed
This way whenever grub-install is called, _grub_ is used instead of shim.
Unfortunately you have found a bug, which will be escalating to shim upstream.
For your current work around to persist, you should also do:
$ sudo dpkg-divert /usr/lib/ shim/shimx64. efi.signed grub/x86_ 64-efi- signed/ grubx64. efi.signed /usr/lib/ shim/shimx64. efi.signed
$ sudo cp /usr/lib/
This way whenever grub-install is called, _grub_ is used instead of shim.
Unfortunately you have found a bug, which will be escalating to shim upstream.