Looking closer at the 19.04 grub.cfg and the post19.10 grub.cfg.new I see that the 19.04 version of update-grub was using 10_linux instead of 10_linux_zfs to generate the grub.cfg.new.
Looking closer at the 19.04 grub.cfg and the post19.10 grub.cfg.new I see that the 19.04 version of update-grub was using 10_linux instead of 10_linux_zfs to generate the grub.cfg.new.