ubiquity installer fails to install grub to correct path

am I missing something or why can't grub always be installed where it is directed to with manual partitioning + preseed?

I use a preseed file for xubuntu 20.04.2 desktop that is devoid of partitioning directives. when "automatic-ubiquity" is booted, after manually partitioning sda selecting /dev/sda1 is ignored and grub is installed into /dev/sda.

added d-i grub-installer/only_debian boolean false and/or d-i grub-installer/with_other_os boolean false to preseed. after manually partitioning sda selecting /dev/sda1 is ignored and grub is installed into /dev/sda.

I hacked a way to dynamically set d-i grub-installer/bootdev string default but that requires booting into the live environment, partitioning the disk and setting the active partition first, then starting "ubiquity -‑automatic" or rebooting with "automatic-ubiquity".

d-i grub-installer/only_debian boolean false
d-i grub-installer/with_other_os boolean false
d-i grub-installer/bootdev string default
d-i partman/early_command string                                               \
 BOOTFLAG=$(lsblk -rpo PARTFLAGS,PKNAME,PATH | grep "0x80" | cut -f 3 -d ' '); \
 if [ $BOOTFLAG ]; then debconf-set grub-installer/bootdev $BOOTFLAG; fi;

testing this works with a vm with a single disk, but may not work as desired on hardware with multiple disks or installing from usb.

d-i grub-installer/skip boolean true seems ignored always. again I hacked a way to make that work, I set d-i grub-installer/bootdev string leaving it empty. manually installing grub in ubiquity/success_command does not work though. inspecting the first few sectors of /dev/sda1, all are 00. the other command completes sucessfully.

d-i grub-installer/only_debian boolean false
d-i grub-installer/with_other_os boolean false
d-i grub-installer/bootdev string 
ubiquity ubiquity/success_command string                                       \
 in-target grub-install -‑force /dev/sda1;                                     \
 echo "tmpfs /tmp tmpfs nosuid,nodev 0 0" >> /target/etc/fstab;

eventually I dropped the partman/early_command method. currently I set grub-installer/bootdev to empty and the use ubiquity/success_command to force install grub to the /target partition and mark it active.

kind of ugly, but it works. tested for sata/nvme/mbr partition table only and separate boot/home/root partitions with ext2/3/4.

d-i grub-installer/only_debian boolean false
d-i grub-installer/with_other_os boolean false
d-i grub-installer/bootdev string 
ubiquity ubiquity/success_command string                                   \
 echo 'tmpfs /tmp tmpfs nosuid,nodev 0 0' >> /target/etc/fstab            ;\
 worker() { grub-install --force --root-directory=/target $2             &&\
 parted -s $3 -- set ${4##*-} boot on; }                                  ;\
 worker $(lsblk -lpo MOUNTPOINT,KNAME,PKNAME,PARTUUID | grep '/target ');

ultimately I think ubiquity needs to be updated to reflect the selections in the gui.