To work around the VM issue:
1. Run
FS0:\EFI\debian\grubx64.efi
in the EFI shell (via a SPICE display, setting up a temporary one if needed) to boot.
2. Log in at the Debian TTY.
3. Follow Debian's instructions to
install GRUB to the "EFI removable media path".
Again, I am not sure this is a TrueNAS regression... perhaps something changed in a recent Debian update. It's just weird I didn't need that workaround with Debian 12 + TrueNAS SCALE 23.10.0.1, but it is needed again with Debian 12 + TrueNAS SCALE 23.10.1.