Woo standard sorting dropdown gives sorting in ABC with SKU set as standard

Hi!
I have added sorting WooCommerce products by SKU as a standard sorting with code in functions.php through the plugin “Theme Customisations”. The sorting works fine when first viewing the store page, but if I click the dropdown to change sorting to let’s say “Price” and then back to “Standard”, the products are sorted Alphabetically instead of by SKU.

I tried switching to the Twentyfifteen theme and there the sorting dropdown works, although the options say “SKU” and not “Standard”.

The code in functions.php:

function sv_add_sku_sorting( $args ) {
	$orderby_value = isset( $_GET['orderby'] ) ? wc_clean( $_GET['orderby'] ) : apply_filters( 'woocommerce_default_catalog_orderby', get_option( 'woocommerce_default_catalog_orderby' ) );

	if ( 'sku' == $orderby_value ) {
		$args['orderby'] = 'meta_value';
		$args['order'] = 'asc'; 
		// ^ lists SKUs alphabetically 0-9, a-z; change to desc for reverse alphabetical
		$args['meta_key'] = '_sku';
	}

		return $args;
	}
	add_filter( 'woocommerce_get_catalog_ordering_args', 'sv_add_sku_sorting' );

function sv_sku_sorting_orderby( $sortby ) {
	$sortby['sku'] = 'Sort by SKU';
	// Change text above as desired; this shows in the sorting dropdown
	return $sortby;
}
add_filter( 'woocommerce_catalog_orderby', 'sv_sku_sorting_orderby' );
add_filter( 'woocommerce_default_catalog_orderby_options', 'sv_sku_sorting_orderby' );