WordPress – Shortcode to output sub-categories with links to posts

I would like to list all subcategories of the main category "Referenzliste" and include links to the posts within those subcategories. This is for a custom post type (referenzen). My script already outputs the subcategories, but it does not find the individual posts even they are published.

function verlinkte_artikel_referenzen($atts) {
    // Attributsbehandlung
    $atts = shortcode_atts(array(
        'parent_cat' => 'Referenzliste', 
        'cpt' => 'referenzen' 
    ), $atts, 'verlinkte_artikel_referenzen');

    // Hole die übergeordnete Kategorie
    $parent_category = get_term_by('name', $atts['parent_cat'], 'category');
    if (!$parent_category) {
        return 'Kategorie nicht gefunden.';
    }

    // Hole die Unterkategorien der übergeordneten Kategorie
    $unterkategorien = get_terms(array(
        'taxonomy' => 'category',
        'child_of' => $parent_category->term_id,
        'hide_empty' => false
    ));

    if (empty($unterkategorien)) {
        return 'Keine Unterkategorien gefunden.';
    }

    $output = '<div class="referenzen-list">';
    foreach ($unterkategorien as $unterkategorie) {
        $output .= '<h2>' . esc_html($unterkategorie->name) . '</h2>';
        $output .= '<ul>';
        
        $query = new WP_Query(array(
            'post_type' => $atts['cpt'],
            'tax_query' => array(
                array(
                    'taxonomy' => 'category',
                    'field' => 'term_id',
                    'terms' => $unterkategorie->term_id
                )
            )
        ));

        if ($query->have_posts()) {
            while ($query->have_posts()) {
                $query->the_post();
                $output .= '<li><a href="' . get_permalink() . '">' . get_the_title() . '</a></li>';
            }
            wp_reset_postdata();
        } else {
            $output .= '<li>Keine Artikel gefunden.</li>';
        }

        $output .= '</ul>';
    }
    $output .= '</div>';

    return $output;
}

add_shortcode('verlinkte_artikel_referenzen', 'verlinkte_artikel_referenzen');