WP_query category__in not working, only pulls from first category

Following Milo‘s answer, I found another workaround that works and i’m more confortable with it.

$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'category__in'    => array(15, 17),
    'posts_per_page' => 4
);

$query = new WP_Query();
$items = $query->query($args);