Created pages not showing up in ‘All Pages’ list

The problem was actually the pages having “no language” and WPML seems to hate this.

So I just deleted all those pages and re-inserted them with the proper WPML language attributes:

// (...)

// Create the page
$customPage['id'] = wp_insert_post( $pageParams );

// Check if WPML parameters are needed
if (has_action('wpml_set_element_language_details')) {
    // Set WPML parameters
    $set_language_args = array(
        'element_id'    => $customPage['id'],
        'element_type'  => apply_filters( 'wpml_element_type', 'page' ),
        'trid'   => FALSE, // new translation id
        'language_code'   => 'it',
        'source_language_code' => NULL
    );
    do_action( 'wpml_set_element_language_details', $set_language_args );
}

This solved my issue.