2025 -> "do_shortcode" doesn´t "DO" Shortcodes, just "Plain Text"

Question

I have a page with a dropdownmenue that is generated by [dynamic_gallery] which is fine, and I see the content I want to see listed, also fine. But if I click on any of the items, a plain text shortcode is being generated, and not the gallery (plugin: FlaGallery) that I need.

here is the full code from my themes ´functions.php:

// Function to create dynamic dropdown for galleries based on date
function dynamic_gallery_dropdown() {
    global $wpdb;
    ob_start();

    // Get all FlaGallery albums from the database
    $table_name = $wpdb->prefix . 'flag_album';
    $albums = $wpdb->get_results("SELECT id, name FROM $table_name");

    // Create an associative array to map album names (DD-MM) to their IDs
    $gallery_map = [];
    foreach ($albums as $album) {
        $gallery_map[$album->name] = $album->id;
    }

    // Get today's date
    $today = new DateTime();

    // Start dropdown HTML
    echo '<select id="galleryDropdown" onchange="loadGallery(this.value)">';
    echo '<option value="">Wähle ein Datum...</option>';

    // Generate next 4 days
    for ($i = 0; $i < 4; $i++) {
        $date = clone $today;
        $date->modify("+$i days");

        // Format for display (e.g., "Dienstag 28.01.2025")
        setlocale(LC_TIME, 'de_DE.UTF-8'); 
        $displayDate = strftime("%A %d.%m.%Y", $date->getTimestamp());

        // Format for FlaGallery lookup (e.g., "28-01")
        $galleryName = $date->format("d-m");

        // Get the corresponding album ID
        if (isset($gallery_map[$galleryName])) {
            $albumID = $gallery_map[$galleryName];
            echo "<option value='$albumID'>$displayDate</option>";
        }
    }

    echo '</select>';
    echo '<div id="galleryContainer"></div>';

    // JavaScript for loading the gallery
    wp_localize_script('jquery', 'ajax_object', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));
    echo "<script>
        function loadGallery(albumID) {
            if (albumID) {
                jQuery.ajax({
                    type: 'POST',
                    url: ajax_object.ajaxurl,
                    data: {
                        'action': 'load_gallery',
                        'albumID': albumID
                    },
                    success: function(data) {
                        // Ensure the gallery content is inserted properly into the container
                        document.getElementById('galleryContainer').innerHTML = data;
                    }
                });
            }
        }
    </script>";

    return ob_get_clean();
}

// Register the dynamic gallery shortcode
add_shortcode('dynamic_gallery', 'dynamic_gallery_dropdown');

// AJAX callback for loading gallery
function load_gallery_callback() {
    if (isset($_POST['albumID'])) {
        $albumID = intval($_POST['albumID']);
        
        // Create the gallery shortcode with the album ID
        $shortcode = '[flagallery album=' . $albumID . ']';

        // Use do_shortcode() to process the gallery shortcode and return the result
        echo do_shortcode('[flagallery album=' . $albumID . ']');
    }
    wp_die(); // End the request
}

add_action('wp_ajax_load_gallery', 'load_gallery_callback');
add_action('wp_ajax_nopriv_load_gallery', 'load_gallery_callback');

// Enqueue jQuery and localize AJAX for frontend use
function enqueue_custom_scripts() {
    wp_enqueue_script('jquery');
    wp_localize_script('jquery', 'ajax_object', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));
}
add_action('wp_enqueue_scripts', 'enqueue_custom_scripts');

tried ajax, JavaScript, with the same result, I really struggle on this one.