WP_Query gives me different results depending on the category order

Question

I have a problem with category__in when I do a WP_Query.

$query_args = array(
    'post_type'  => 'posts',
    'nopaging'   => true,
);

$categories = array(1,8); // If I use this, I get 3 posts
$categories = array(8, 1); // If I use this, I get 2 posts
$query_args['category__in'] = $categories;

$query = new WP_Query($query_args);

Apparently I get different results when I pass it "array(1,8)" than when I pass it "array(8,1)".

Is the order important??? Or is this a bug??

UPDATE

This is the request for the case array(1,8)

string(571) "SELECT   wp_posts.* FROM wp_posts  LEFT JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)  LEFT JOIN wp_term_relationships AS tt1 ON (wp_posts.ID = tt1.object_id)  LEFT JOIN wp_term_relationships AS tt2 ON (wp_posts.ID = tt2.object_id) WHERE 1=1  AND ( 
  wp_term_relationships.term_taxonomy_id IN (1) 
  AND 
  tt1.term_taxonomy_id IN (1) 
  AND 
  tt2.term_taxonomy_id IN (1,8)
) AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'publish' OR wp_posts.post_status = 'private') GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC "

and for array(8,1)

string(571) "SELECT   wp_posts.* FROM wp_posts  LEFT JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)  LEFT JOIN wp_term_relationships AS tt1 ON (wp_posts.ID = tt1.object_id)  LEFT JOIN wp_term_relationships AS tt2 ON (wp_posts.ID = tt2.object_id) WHERE 1=1  AND ( 
  wp_term_relationships.term_taxonomy_id IN (8) 
  AND 
  tt1.term_taxonomy_id IN (8) 
  AND 
  tt2.term_taxonomy_id IN (1,8)
) AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'publish' OR wp_posts.post_status = 'private') GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC "

Answer 1

I solved it the following way

$query_args['tax_query'] = array(
        'relation' => 'OR',
        array(
            'taxonomy' => 'category',
            'field'    => 'id',
            'terms'    => $categories,
        ),
    );

Apparently I had to explicitely say that I was filtering through the taxonomy "category". I could see that thanks to the comment from @birgire. Thanks man!

Update

As petition, this is the request:

SELECT wp_posts.*
FROM wp_posts LEFT JOIN wp_term_relationships
ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE 1=1 AND (wp_term_relationships.term_taxonomy_id IN (1,8)) 
    AND wp_posts.post_type = 'post'
    AND (wp_posts.post_status = 'publish'
        OR wp_posts.post_status = 'private')
GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC