BUG? WP_Query() does not return the same result as query_posts()

Hello!

I’m running WordPress 3.8.1 and I’m having problems using a custom WP_Query().

I’m using an array with these arguments:

$args = array(
  'tag_slug__and' => array('usa', 'china'),
  'category__and' => array(211, 420),
  'posts_per_page' => '-1',
  's' => 'money',
);

The following code works:

$GLOBALS['wp_query'] = new WP_Query();
return $GLOBALS['wp_query']->query($args);

Whereas this does not work:

$obj = new WP_Query();
$posts = $obj->query($args);
return $posts;

I var_dumped each object and they look exactly the same, except for one difference – the first one contains has a post in the “posts” array whereas the second one is empty.

The first code above is taken straight out of query_posts().

Do I have to use $GLOBALS to get this to work?

I should add that the “s” key plays a major role. If I remove the row that contains the search string, the second code snip works as well.

This feels like a WP bug..