Converting hierarchical pages to hierarchical CPT

I am trying to convert an existing hierarchy of around 300 pages to custom post types, in order to be able to manage them better.

My problem is however, that once I convert them by using the plugin, after clicking “save”, WordPress adds a number at the end of the slug, as if they were duplicates. I tried refreshing the permalink structure as well.

How can I convert hierarchical pages to hierarchical CPT?

Here is the code for registering the custom post type:

$args = array(
    'show_in_rest' => true,
    'public' => true, 
    'publicly_queryable'  => true,
    'show_ui'             => true,
    'menu_position'       => 4,
    'hierarchical'        => true,
    'has_archive'         => false,
    'query_var'           => 'holiday',
    'map_meta_cap'        => true,
    'rewrite' => array(
        'slug'       => 'holiday',
        'with_front' => true, 
        'pages'      => true,
        'feeds'      => true,
        'ep_mask'    => EP_PERMALINK
    ),
    'supports' => array(
        'title',
        'editor',
        'excerpt',
        'author',
        'thumbnail',
        'revisions',
        'page-attributes'
    )
);

register_post_type(
    'holiday',
    $args
);

The page I need help with: [log in to see the link]