Issue adding elements to an array in post meta using REST API

Hi,

I have a problem adding elements to an array of user_ids in post meta using the REST API. This works well when not integrated with the REST API, but when I do it through the API, the new value I add to the array with the list of IDs in post meta always overwrite the previous value, rather than add it in the list. I am sure this is something really simple: your help would really GREAT!!!

Thanks

register_rest_field(
    'post',
    'nb_post_registered_user_liked',
    array(
        'get_callback'      => 'nb_get_post_registered_user_liked',
        'update_callback'   => 'nb_update_post_registered_user_liked',
        'delete_callback'   => null,
        'schema'            => $nb_post_registered_user_liked_schema // defined separately
        )

function nb_get_post_registered_user_liked( $object, $field_name, $request ) {
    return get_post_meta( $object['id'], 'nb_post_liked_user_ids', true ); // to test post meta
}

function nb_update_post_registered_user_liked( $value, $object, $field_name ) {
    $nb_post_liked_user_ids_meta = null;
    $user_key = null;
    
    $user_id = get_current_user_id();
    $nb_post_liked_user_ids_meta = get_post_meta( $object->ID, 'nb2_post_liked_user_ids', true );
    if ( count( $nb_post_liked_user_ids_meta ) != 0 ) {
        $nb_post_liked_user_ids_meta = $nb_post_liked_user_ids_meta[0];
    }
    if ( !is_array( $nbu_post_liked_user_ids ) ) {
	$nb_post_liked_user_ids_meta = array();
    }

    $user_key = array_search( $user_id, $nb_post_liked_user_ids_meta );
    if ( !$user_key ) {
        $nb_post_liked_user_ids_meta['user-' . $user_id] = $user_id;
        $nb_post_liked_user_ids_meta[] = $user_id; // tried also this way...
        update_post_meta( $object->ID, 'nb_post_liked_user_ids', $nb_post_liked_user_ids_meta );
        return nb_post_liked_user_ids_meta; // to test
    };
}