Comment # 6 on bug 1101756 from Josef Reidinger

I worry that working with ESP partition is done a layer lower then
yast2-bootloader. Yast2 just checks that there is ESP partition, but do not
modify it. It only calls grub2-install and grub2-mkconfig.

But I agree that this change of GRUB_DISTRIBUTOR is a bug that should be fixed.
So your manual edit of default/grub should be respected.