How get "Widget Content Blocks" plugin's internal search to work in wp-admin panel?

Question

I'm using Widget Content Blocks plugin for quite a while. Since there are over 100 items now I need to search a keyword. There is an inbuilt searchbar but none of any keywords gives a result.

Is it a plugin issue or an issue of wordpress admin panel? I wrote to the plugin support a while ago but didn't get an answer yet.

Here is part of the code in "plugin.php":

 * Register the post type used for the widget blocks
 */
function wywi_register_post_type()  {
    $labels = array(
        'name' => __('Widget Blocks', 'wysiwyg-widgets'),
        'singular_name' => __('Widget Block', 'wysiwyg-widgets'),
        'add_new' => __('New Widget Block', 'wysiwyg-widgets'),
        'add_new_item' => __('Add New Widget Block', 'wysiwyg-widgets'),
        'edit_item' => __('Edit Widget Block', 'wysiwyg-widgets'),
        'new_item' => __('New Widget Block', 'wysiwyg-widgets'),
        'all_items' => __('All Widget Blocks', 'wysiwyg-widgets'),
        'view_item' => __('View Widget Block', 'wysiwyg-widgets'),
        'search_items' => __('Search Widget Blocks', 'wysiwyg-widgets'),
        'not_found' =>  __('No widget blocks found', 'wysiwyg-widgets'),
        'not_found_in_trash' => __('No widget blocks found in Trash', 'wysiwyg-widgets'), 
        'menu_name' => __('Widget Blocks', 'wysiwyg-widgets')
      );
    $args = array(
        'public' => false,
        'show_ui' => true,
        'labels' => $labels,
        'supports' => array('title', 'editor')
    );

    register_post_type( 'wysiwyg-widget', $args );
}

add_action('init', 'wywi_register_post_type');

Any ideas how to fix it? Or at least investigate within wordpress config?

EDIT:// I checked that the same search bar within "All posts" is working. So most probably it is an issue of the plugin itself. Am I right?