wpquery

    $search = sanitize_text_field($_POST['s']);
    $region = (isset($_POST['region']) ? sanitize_text_field($_POST['region']) : '' );
    $post_type = sanitize_text_field($_POST['post_type']);

    $activite = array();
    foreach($_POST as $p => $i){
        if(strpos($p,'activite') !== false){
            array_push($activite, $i);
        }
    }

    $service = array();
    foreach($_POST as $p => $i){
        if(strpos($p,'service') !== false){
            array_push($service, $i);
        }
    }

    //echo $search . PHP_EOL . $region . PHP_EOL . $activite . PHP_EOL . $service . PHP_EOL;
    $args = array(
        'post_type' => $post_type,
        's' => $search,
        'order' => 'asc',
        'orderby' => 'title',
        'posts_per_page' => '-1',
        'tax_query' => array('relation' => 'AND')
    );
    if($region != ''){
        array_push($args['tax_query'], array('taxonomy' => 'region',
                                            'field' => 'slug',
                                            'terms' => $region)
                    );
    }


    // These are the faulty taxonomies
    if(count($activite) > 0){
        array_push($args['tax_query'], array('taxonomy' => 'activite',
            'field' => 'slug',
            'terms' => $activite,
            'operator' => 'IN'));
    }

    if(count($service) > 0){
        array_push($args['tax_query'], array('taxonomy' => 'service',
            'field' => 'slug',
            'terms' => $service));
    }

    $query = new WP_Query( $args );
    $results = $query->get_posts();

-------------------------------------
Args dump with 1 param checked

array(6) {
  ["post_type"]=>
  string(8) "vignoble"
  ["s"]=>
  string(0) ""
  ["order"]=>
  string(3) "asc"
  ["orderby"]=>
  string(5) "title"
  ["posts_per_page"]=>
  string(2) "-1"
  ["tax_query"]=>
  array(2) {
    ["relation"]=>
    string(3) "AND"
    [0]=>
    array(4) {
      ["taxonomy"]=>
      string(8) "activite"
      ["field"]=>
      string(4) "slug"
      ["terms"]=>
      array(1) {
        [0]=>
        string(9) "activite1"
      }
      ["operator"]=>
      string(2) "IN"
    }
  }
}

--------------------------------------
Args dump with 2 params

array(6) {
  ["post_type"]=>
  string(8) "vignoble"
  ["s"]=>
  string(0) ""
  ["order"]=>
  string(3) "asc"
  ["orderby"]=>
  string(5) "title"
  ["posts_per_page"]=>
  string(2) "-1"
  ["tax_query"]=>
  array(2) {
    ["relation"]=>
    string(3) "AND"
    [0]=>
    array(4) {
      ["taxonomy"]=>
      string(8) "activite"
      ["field"]=>
      string(4) "slug"
      ["terms"]=>
      array(2) {
        [0]=>
        string(9) "activite1"
        [1]=>
        string(9) "activite2"
      }
      ["operator"]=>
      string(2) "IN"
    }
  }
}