How do I use a plugin's shortcode attribute value in another function?

I have a plugin as a class, where I am using a shortcode attribute to populate the $outlet variable. However, despite sanitize_text_field($atts['outlet']) returning the expected string , $this->outlet = sanitize_text_field($atts['outlet']) isn't updating the value of $outlet. If I set a default value for the class's $outlet initially, it works as expected below in the example_callback(). Everything else with the plugin works as expected. It's just wp_news_shortcode() not assigning the $attr value...

In a word, I need the shortcode attribute value in the ajax_callback() function—is there something I've overlooked?

class Example _Search {
       
        private static $instance = null;

        public $outlet;

        public function __construct() {
            $this->settings = array(
                'plugin_path' => plugin_dir_path(__FILE__),
                'plugin_url' => plugin_dir_url(__FILE__),
                'plugin_base' => dirname(plugin_basename(__FILE__)),
                'plugin_base_url' => plugin_basename(__FILE__),
                'plugin_file' => __FILE__,
                'plugin_version' => '1.0.0',
            );

            $this->run_plugin();
        }

         /**
         * Singleton.
         *
         * @return self Main instance.
         */
        public static function init() {
            if (is_null(self::$instance)) {
                self::$instance = new self();
            }
            return self::$instance;
        }

        /**
         * Main plugin function.
         *
         * @since 1.0.0
         */
        public function run_plugin() {
            add_action('wp_ajax_example_callback', array($this, 'example_callback'));
            add_action('wp_ajax_nopriv_example_callback', array($this, 'example_callback'));
            add_shortcode('wp_example_search', array($this, 'wp_example_shortcode'));
        }

       

        /**
         * Example Search shortcode. Adds form to page and stores "outlet" attribute to options.
         *
         * @since 1.0.0
         * @param array $atts An associative array of attributes.
         * @return string
         */
        public function wp_example_shortcode($atts) {
            $atts = shortcode_atts(
                array(
                    'outlet' => 'google',
                ),
                $atts
            );
            $this->outlet = sanitize_text_field($atts['outlet']);

            $content = '';
            $view = $this->get_view_path('form.php');
            if (file_exists($view)) {
                ob_start();
                include $view;
                $content = ob_get_clean();
            }
            return $content;
        }

        /**
         * Get the path to view.
         *
         * @param string $view_name View name.
         * @param string|boolean $sub_dir_name The sub-directory.
         * @return string
         */
        public function get_view_path($view_name, $sub_dir_name = false) {
            $path = $rel_path = '';
            $plugin_base = 'wp-example-search';
            if (!empty($sub_dir_name)) {
                $rel_path .= "/{$sub_dir_name}";
            }
            $rel_path .= "/{$view_name}";
            $path = $this->settings['plugin_path'] . 'views' . $rel_path;
            return $path;
        }

        /**
         * Handle AJAX request.
         */
        public function example_callback() {

            $_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_STRING);

            if (isset($_POST['data'])) {
                $form_data = ($_POST['data']);
                $keyword = trim($form_data['keyword']);

                $endpoint = 'https://example.com?q=' . rawurlencode($keyword) . '&domains=' . 
                $this->outlet . '.com';
                
                ];
                $response = wp_remote_get($endpoint);
                $response_body = wp_remote_retrieve_body($response);
                $result = json_decode($response_body);
                $result_items = $result->items;

                if (is_array($result) && !is_wp_error($result)) {
                    $error_message = $result->get_error_message();
                    echo "Something went wrong: $error_message";
                }  else {
                    $content = '';
                    $view = $this->get_view_path('article.php');
                    if (file_exists($view)) {
                        ob_start();
                        include $view;
                        $content = ob_get_clean();
                    }
                    echo $content;
                }
            }

            die();

        }
    }

    Example_Search::init();