WooCommerce programtically add variations to product last variation is not filled

Question

We are importing products in our WooCommerce shop with an API. All is working but we have a problem with the variations we are importing for a product. Everything is importing correctly but the last variation is set to Every size... while it should be 11/XXL. If I click on the variation which is set to Every size... all the data is filled in but it is not automatically setting the variation to 11/XXL while all other variations are set automatically the correct way.

Here is the function we are using to add the variations.

/**
 * Create a product variation
 */
function create_variation($product_id, $variation_data) {    
    // Get variable product parent
    $product = wc_get_product($product_id);

    $variation_post = array(
        'post_title'    => $product->get_name(),
        'post_name'     => 'product-'.$product_id.'-variation',
        'post_status'   => 'publish',
        'post_parent'   => $product_id,
        'post_type'     => 'product_variation',
        'guid'          => $product->get_permalink()
    );

    // Create product variation
    $variation_id = wp_insert_post($variation_post);

    // Get WC_Product instance of variation
    $variation = new WC_Product_Variation($variation_id);

    // Iterate through variation attributes
    foreach($variation_data['attributes'] as $attribute => $term_name)
    {
        $taxonomy = 'pa_'.$attribute;

        // Create taxonomy if it doesnt exist
        if(!taxonomy_exists( $taxonomy )) {
            register_taxonomy(
                $taxonomy,
                'product_variation',
                array(
                    'hierarchical' => false,
                    'label' => ucfirst($attribute),
                    'query_var' => true,
                    'rewrite' => array('slug' => sanitize_title($attribute)),
                )
            );
        }

        // Create term if it doesnt exist
        if(!term_exists($term_name, $taxonomy))
            wp_insert_term($term_name, $taxonomy);

        $term_slug = get_term_by('name', $term_name, $taxonomy)->slug;

        // Set/save the attribute data in the product variation
        update_post_meta($variation_id, 'attribute_'.$taxonomy, $term_slug);
 
        // Get the post Term names from the parent variable product.
        $post_term_names = wp_get_post_terms($product_id, $taxonomy, array('fields' => 'names'));

        // Check if the post term exists and if not, set it in the parent product
        if(!in_array($term_name, $post_term_names))
            wp_set_post_terms($product_id, $term_name, $taxonomy, true);
    }

    ## Set/save other data
    //SKU
    if(!empty($variation_data['sku']))
        $variation->set_sku($variation_data['sku']);

    //Prices
    if(empty($variation_data['sale_price'])) {
        $variation->set_price($variation_data['regular_price']);
    }else {
        $variation->set_price($variation_data['sale_price']);
        $variation->set_sale_price($variation_data['sale_price']);
    }
    $variation->set_regular_price($variation_data['regular_price']);

    // Stock
    if(!empty($variation_data['stock_qty'])) {
        $variation->set_stock_quantity($variation_data['stock_qty']);
        $variation->set_manage_stock(true);
        $variation->set_stock_status('');
    }else {
        $variation->set_manage_stock(false);
    }
    $variation->set_weight('');

    $variation->save();
}

Here is the $variation_data

Array
(
    [attributes] => Array
        (
            [maat] => 7/S
        )

    [sku] => 14120007
    [regular_price] => 1.53
)
Array
(
    [attributes] => Array
        (
            [maat] => 6/XS
        )

    [sku] => 14120006
    [regular_price] => 1.98
)
Array
(
    [attributes] => Array
        (
            [maat] => 8/M
        )

    [sku] => 14120008
    [regular_price] => 1.53
)
Array
(
    [attributes] => Array
        (
            [maat] => 9/L
        )

    [sku] => 14120009
    [regular_price] => 1.53
)
Array
(
    [attributes] => Array
        (
            [maat] => 10/XL
        )

    [sku] => 14120010
    [regular_price] => 1.53
)
Array
(
    [attributes] => Array
        (
            [maat] => 11/XXL
        )

    [sku] => 14120011
    [regular_price] => 1.53
)

Here is the output in the back-end of WooCommerce

As you can see the variation 11/XXL is set as Every size... but it should be 11/XXL.

Does anyone know how to fix this so the variation is set as 11/XXL instead of Every size...

Thanks for your time!