informatiq

This code snippet fixed it for us. Add to your functions.php:

// Hook into the wpseo_sitemap_index action
add_action('wpseo_sitemap_index', 'custom_modify_sitemap_output');

function custom_modify_sitemap_output() {
    // Start output buffering
    ob_start('custom_modify_sitemap_buffer');

    // Call the Yoast SEO sitemap index function
    do_action('wpseo_xml_sitemap_index', 'always', true);

    // End output buffering and get the modified sitemap content
    $modified_sitemap_content = ob_get_clean();

    // Log the original and modified sitemap content
    error_log('Original sitemap content: ' . ob_get_clean());
    error_log('Modified sitemap content: ' . $modified_sitemap_content);

    // Output the modified sitemap content
    echo $modified_sitemap_content;
}

function custom_modify_sitemap_buffer($buffer) {
    // Remove any whitespace before the XML declaration
    $buffer = preg_replace('/^\s+/', '', $buffer);

    // Ensure the XML declaration is at the beginning
    if (strpos($buffer, '<?xml') !== 0) {
        $buffer = '<?xml version="1.0" encoding="UTF-8"?>' . "\n" . $buffer;
    }

    return $buffer;
}