You’re using get_post_meta() instead on get_field(). ACF only stores the user ID, so this is what will be returned by get_post_meta(). If you use get_field() ACF will return a user object, array, or ID depending on your field settings. In either case you need to get the user object and show the parts of it you want. https://codex.wordpress.org/Function_Reference/get_userdata
