Secondary loop pagination on custom post type single post gets redirected to first page

I would like to output related posts on a custom post-type single page. Everything seems to be set up correctly. When I hover over my pagination links I see http://example.com/post-type-slug/post-slug/page/2/. When I click I go to that page, but then get redirected back to the first page: http://example.com/post-type-slug/post-slug/

This is how I’ve set up my post type:

register_post_type( 'posttype',
    array(
        'rewrite' => array(
            'slug' => 'post-type-slug',
            'with_front' => false,
        ),
        'public' => true,
        'publicly_queryable' => true,
        'exclude_from_search' => true,
        'has_archive' => false,
        'menu_position'=>5,
    )
);

This is how I am doing my WP_query on the single page

    $queryArgs = array( 
        'posts_per_page' => 3,
    );
    $queryArgs['paged'] = get_query_var( 'page' ) ? get_query_var('page') : 1; 
    $customQuery = new WP_Query( $queryArgs );

For my pagination I am using the method described here: https://wordpress.stackexchange.com/a/172818/10595

Why do I get redirected to the index? What can I do to prevent this.

I checked out some things:
When I disable pretty permalinks the pagination links say http://example.com/? posttype=post-slug&paged=2. When I manually change that to http://example.com/? posttype=post-slug&page=2 everything works as I want it to. How can I alter my pagination to do this for me?

I found the answer. I had searched high and low yesterday, but today I finally found it: https://wordpress.stackexchange.com/a/143837/10595

add_action('template_redirect', function() {
  if ( is_singular('posttype') ) {
    global $wp_query;
    $page = (int) $wp_query->get('page');
    if ( $page > 1 ) {
      // convert 'page' to 'paged'
      $query->set( 'page', 1 );
      $query->set( 'paged', $page );
    }
    // prevent redirect
    remove_action( 'template_redirect', 'redirect_canonical' );
  }
}, 0 ); // on priority 0 to be able removing 'redirect_canonical' added with priority 10

This, copied to my functions.php, makes WordPress recognize the paged parameter on single posttype entries.