I have hit this behavior in the debian-buster-alpha4 installer as well. Exact same issue. I setup a RAID1 in the installer and then installed the base system. In the Grub dialog I choose the first disk, let's call it /dev/sda to install Grub on. Then the installer goes and tries to install grub on /dev/md. Manually entering /dev/sda is a workaround and I confirm that the newly installed system boots afterwords.
-- ~ ry
