Custom select disappears after filtering on WooCommerce admin orders page

I added a filter by product to WooCommerce admin orders page, using:

function add_my_product_filter_to_orders() {
    global $typenow;

    if ($typenow !== 'shop_order') return;

    if (!current_user_can('administrator')) { return; }

    $args = array(
        'post_type'      => 'product',
        'posts_per_page' => -1,
        'tax_query'      => array(
            array(
                'taxonomy' => 'product_cat',
                'field'    => 'slug',
                'terms'    => 'test',
            ),
        ),
    );

    $products = get_posts($args);
    if (!$products) return;

    $selected_product = isset($_GET['my_product_filter']) ? esc_attr($_GET['my_product_filter']) : '';

    echo '<select name="my_product_filter">';
    echo '<option value="">filter by product</option>';
    
    foreach ($products as $product) {
        echo '<option value="' . $product->ID . '" ' . selected($selected_product, $product->ID, false) . '>' . get_the_title($product->ID) . '</option>';
    }

    echo '</select>';
}
add_action('restrict_manage_posts', 'add_my_product_filter_to_orders', 25);

We used pre_get_posts hook for filtration.

function filter_orders_by_my_product($query) {
    global $pagenow, $typenow, $wpdb;

    if ($pagenow !== 'edit.php' || $typenow !== 'shop_order' || empty($_GET['my_product_filter'])) {
        return;
    }

    $product_id = intval($_GET['my_product_filter']);

    $order_ids = $wpdb->get_col($wpdb->prepare("
        SELECT order_id FROM {$wpdb->prefix}woocommerce_order_items oi
        JOIN {$wpdb->prefix}woocommerce_order_itemmeta oim ON oi.order_item_id = oim.order_item_id
        WHERE oim.meta_key = '_product_id' AND oim.meta_value = %d
    ", $product_id));

    if (!empty($order_ids)) {
        $query->query_vars['post__in'] = $order_ids;
    } else {
        $query->query_vars['post__in'] = array(0);
    }
}
add_action('pre_get_posts', 'filter_orders_by_my_product');

On start the dropdown is displayed and everything is fine, but after filtering, it disappears and $products = get_posts($args) returns an empty array after filtration. How to fix that issue?

Instead of making an additional query to filter orders, it's much better and lightweight to change the original orders query using posts_request hook...

It also solves the issue where your custom filter dropdown disappears.

Try the following revised code:

add_action('restrict_manage_posts', 'add_custom_product_filter_to_orders', 25);
function add_custom_product_filter_to_orders() {
    global $typenow, $pagenow;

    if ( 'edit.php' === $pagenow && 'shop_order' === $typenow ) :

    $products = get_posts( array(
        'post_type'      => 'product',
        'posts_per_page' => -1,
        'tax_query'      => array( array(
            'taxonomy' => 'product_cat',
            'field'    => 'slug',
            'terms'    => 'test',
        ) ),
    ) );

    if ( !$products ) return;

    $selected_product = isset($_GET['product_id']) ? absint($_GET['product_id']) : '';

    printf('<select name="product_id">
    <option value="">%s</option>', esc_html__('All products') );
    
    foreach ($products as $product) {
        printf('<option value="%d" %s>%s</option>', $product->ID,  selected($selected_product, $product->ID, false), get_the_title($product->ID) );
    }

    echo '</select>';
    endif;
}

// Change the main SQL query
add_filter('posts_request', 'change_admin_orders_request_by_product', 20, 2);
function change_admin_orders_request_by_product($sql, $query) {
    global $typenow, $pagenow, $wpdb;

    if ( is_admin() && 'edit.php' === $pagenow && 'shop_order' === $typenow
    && isset($_GET['product_id']) && ! empty($_GET['product_id']) ) {
        $join = " INNER JOIN {$wpdb->prefix}woocommerce_order_items AS oi
            ON {$wpdb->prefix}posts.ID = oi.order_id
            INNER JOIN {$wpdb->prefix}woocommerce_order_itemmeta AS oim
            ON oi.order_item_id = oim.order_item_id";
        $and = $wpdb->prepare(" 
            AND oim.meta_key = '_product_id' AND oim.meta_value = %d", absint($_GET['product_id']) );
            
        $sql = str_replace( "FROM wp_posts", "FROM wp_posts  {$join}", $sql );
        $sql = str_replace( "WHERE 1=1  AND", "WHERE 1=1  {$and} AND", $sql );
    }
    return $sql;
}

Code goes in functions.php file of your child theme (or in a plugin). Tested and works.

Filtering orders for a product with results:

Filtering orders for a product without results:

The custom dropdown filter is still displayed.

Note: This code will not when High Performance Order Storage is enabled.