To me this doesn't look like a partman issue, since the grub-installer is practically ignoring the bootdev set in the preseed file, ("debconf: --> SUBST grub-installer/progress/step_install_loader BOOTDEV /dev/sda"), and since I am also forgoing the partman partitioning (also a work-around)
Same issue for Xenial.
To me this doesn't look like a partman issue, since the grub-installer is practically ignoring the bootdev set in the preseed file, ("debconf: --> SUBST grub-installer/ progress/ step_install_ loader BOOTDEV /dev/sda"), and since I am also forgoing the partman partitioning (also a work-around)