I found my problem.
It was just a problem from my search php script! Without trim( $_REQUEST['term'] )
it will not correctly filter results.
Now like this it works perfectly:
function search_autocomplete(){
$posts = get_posts( array(
's' => trim( $_REQUEST['term'] ), 'post_type' => array('post', 'portfolio')
) );
$suggestions=array();
global $post;
$args = array('post_type' => array('post', 'portfolio'), 'posts_per_page' => 10, 'post_status' => 'publish');
$post = get_posts($args);
foreach ($posts as $post):
setup_postdata($post);
$suggestion = array();
$suggestion['label'] = esc_html($post->post_title);
$suggestion['link'] = get_permalink();
$suggestions[]= $suggestion;
endforeach;
$response = $_GET["callback"] . "(" . json_encode($suggestions) . ")";
echo $response;
exit;
}