and which one of those fields is the post object field?
A post object field stores a post ID and does not store the post title of the post that is selected. If you want to store information about that other post then you will need to get that other post, it is also a string in $)POST and not an INT
$post = get_post(intval($_POST['acf']['key_for_post_object_field']));
// use title of post
$value = $post->post_title;
