Wordpress - Broken UI of CMB2/cmb2-attached-posts library

Question

I have implemented the CMB2/cmb2-attached-post library in my wordpress installation and created a custom post type in which to use it. I have implemented the example described in the example-field-setup-php, specifically, in this way:

$example_meta = new_cmb2_box( array(
        'id'           => 'cmb2_attached_posts_field',
        'title'        => __( 'Attached Posts', 'yourtextdomain' ),
        'object_types' => array( 'luogo' ), // Post type
        'context'      => 'normal',
        'priority'     => 'high',
        'show_names'   => true, // Show field names on the left
    ) );

    $example_meta->add_field( array(
        'name'    => __( 'Attached Posts', 'yourtextdomain' ),
        'desc'    => __( 'Drag ...', 'yourtextdomain' ),
        'id'      => 'attached_cmb2_attached_posts',
        'type'    => 'custom_attached_posts',
        'column'  => true,
        'options' => array(
            'show_thumbnails' => true,
            'filter_boxes'    => true,
            'query_args'      => array(
                'posts_per_page' => 10,
                'post_type'      => 'page',
            ),
        ),
    ) );

    $example_meta->add_field( array(
        'name'    => __( 'Attached Users', 'yourtextdomain' ),
        'desc'    => __( 'Drag ...', 'yourtextdomain' ),
        'id'      => 'attached_cmb2_attached_users',
        'type'    => 'custom_attached_posts',
        'column'  => true,
        'options' => array(
            'show_thumbnails' => true,
            'filter_boxes'    => true,
            'query_users'     => true,
        ),
    ) );

The result I get graphically is this:

Which is visibly broken and malfunctioning. What I got as a result is the following: (as shown on the library repo).

Why do I get a broken result if the library has been integrated as per the documentation? Is something missing?

What I have done for use is: Installing the library with composer and the inclusion of the library in the .php file where I create the custom post type:

require_once ( get_template_directory() . /vendor/webdevstudios/cmb2-attached-posts/init.php' );