Well, that is weird because that feature works perfectly for me. As a last resort you can manually override the default grid setting with this code:
add_filter( 'enlightenment_default_grid', 'enlightenment_filter_default_grid' );
function enlightenment_filter_default_grid( $grid ) {
return 'threecol';
}
You should add it to your child theme’s functions.php file so you don’t lose it on next update.