Creating WooCommerce product variations programatically not bounding a particular attribute to a particular variation

Question

/**
 * Plugin Name:     ERP - WooCommerce Sync
 * Description:     Erp Sync
 */

// Exit if accessed directly
if(!defined('ABSPATH')) {
    exit;
}

// Getting the saved API response
$api = file_get_contents(plugin_dir_path( _FILE_ ) . 'api.json', 'r');
$api_products = json_decode($api);
$product_ids = [336, 337];

add_action( 'wp_loaded', function() use ($api_products, $product_ids) {
   foreach ($product_ids as $product_id)
       create_update_variations($api_products, $product_id);
});

/**
 * Creating/updating attributes and variations
 */
function create_update_variations($api_products, $product_id) {
    $size_variations = [];
    $fusion_variations = [];

    // Taking nd/single order
    foreach ($api_products[1]->price_all->rows as $api_product) {
        if (str_contains($api_product[0], 'nd') && $api_product[2] == 'single') {
            $fusion_variations[] = $api_product[1];
            $size_variations[] = $api_product[5];
        }
    }

    $fusion_variations = array_unique($fusion_variations);

    // Loading attributes
    $product = wc_get_product($product_id);
    $attributes = $product->get_attributes();

    // Creating or updating sizes
    if (!isset($attributes['Size'])) {
        $size_attribute = new WC_Product_Attribute();
        $size_attribute->set_id(0);
        $size_attribute->set_name('Size');
        $size_attribute->set_options($size_variations);
        $size_attribute->set_visible(true);
        $size_attribute->set_variation(true);
        $attributes['Size'] = $size_attribute;
    } else {
        $size_attribute = $attributes['Size'];
        $size_attribute->set_options($size_variations);
    }

    // Creating or updating fusions
    if (!isset($attributes['Fusion'])) {
        $fusion_attribute = new WC_Product_Attribute();
        $fusion_attribute->set_id(0);
        $fusion_attribute->set_name('Fusion');
        $fusion_attribute->set_options($fusion_variations);
        $fusion_attribute->set_visible(true);
        $fusion_attribute->set_variation(true);
        $attributes['Fusion'] = $fusion_attribute;
    } else {
        $fusion_attribute = $attributes['Fusion'];
        $fusion_attribute->set_options($fusion_variations);
    }

    $product->set_attributes($attributes);
    $product->save();

    // Creating variations
    $variable_product = wc_get_product($product_id);
    $existing_variations = $variable_product->get_children();

    foreach ($size_variations as $size) {
        foreach ($fusion_variations as $fusion) {
            // Check if a variation with these attributes already exists
            $variation_exists = false;
            foreach ($existing_variations as $variation_id) {
                $variation = wc_get_product($variation_id);
                $variation_attributes = $variation->get_attributes();
                if (isset($variation_attributes['Size']) && isset($variation_attributes['Fusion']) && $variation_attributes['Size'] == $size && $variation_attributes['Fusion'] == $fusion) {
                    $variation_exists = true;
                    break;
                }
            }

            // If the variation doesn't exist, create it
            if (!$variation_exists) {
                $variation = new WC_Product_Variation();
                $variation->set_regular_price(20);
                $variation->set_attributes(array(
                    'Fusion' => $fusion,
                    'Size' => $size
                ));
                $variation->set_parent_id($product_id);
                $variation->save();
            }
        }
    }
}```

This code successfully creating variations, but not bounding their attributes. Here is the "api.json" file code:

```[
  {
    "client": "TSS",
    "manager": "Mike",
    "unit": "М14",
    "category_m14store": ""
  },
  {
    "price_all": {
      "columns": [
        {
          "text": "Name",
          "type": "string"
        },
        {
          "text": "Fusion",
          "type": "string"
        },
        {
          "text": "PriceType",
          "type": "string"
        },
        {
          "text": "ProdType",
          "type": "string"
        },
        {
          "text": "Price",
          "type": "string"
        },
        {
          "text": "Size",
          "type": "string"
        }
      ],
      "rows": [
        [
          "Nd - 31,75",
          "oen",
          "single",
          "ball oen;oen-mg",
          "25",
          "31,75"
        ],
        [
          "Nd - 32,00",
          "oen",
          "single",
          "ball oen;oen-mg",
          "25",
          "32"
        ]
      ],
      "type": "table"
    }
  }
]```