ACF order by date

function my_relationship_query( $args, $field, $post_id ) {
	
    // only show children of the current post being edited
    $args['post_date'] = 'DESC';
	
	
	// return
    return $args;
    
}


// filter for every field
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);

Try setting the params as

 $args['orderby'] = 'post_date';
 $args['order'] = 'DESC';