The user field returns an array of user information, similar to what the WP function get_users() does. I don’t think there has ever been documentation on the user field. You’ve got to do some experimenting to see what’s there.
To see what is in the returned value use
echo '<pre>'; print_r(get_sub_field('author')); echo '</pre>';
to show the values you need to use
$users = get_sub_field('author');
and then show what you need. It might be a nested array, I’m not sure.
