Code used to check the BuddyPress WP xProfile field type works as expected, see https://buddypress.org/support/topic/wordpress-specific-xprofile-meta-fields-not-saving/

bp-custom.php

<?php
// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

function maelscuttle_twitter_xprofile( $methods = array() ) {
	$methods['twitter'] = __( 'Twitter', 'maelscuttle-textdomain' );

	return $methods;
}
add_filter( 'user_contactmethods', 'maelscuttle_twitter_xprofile' );