WP_query category__in not working, only pulls from first category

I wrote a WP_query and the behaviour is weird. I tried almost everything but that's not working. I found a solution but i try to understand.

The following queries always returns posts from the first caregory (id : 15, slug : slug1).

$args = array( 'post_type' => 'post', 'post_status' => 'publish', 'category__in' => array(15, 17), 'posts_per_page' => 4 ); $query = new WP_Query($args); $items = $query->get_posts(); 

OR

$args = array( 'post_type' => 'post', 'post_status' => 'publish', 'cat' => '15,17', 'posts_per_page' => 4, ); $query = new WP_Query($args); $items = $query->get_posts(); 

OR

$args = array( 'post_type' => 'post', 'post_status' => 'publish', 'category_name' => 'slug1,slug2', 'posts_per_page' => 4, ); $query = new WP_Query($args); $items = $query->get_posts(); 

OR

$args = array( 'post_type' => 'post', 'post_status' => 'publish', 'posts_per_page' => 4, 'tax_query' => array( array( 'taxonomy' => 'category', 'field' => 'term_id', 'terms' => array(15,17), ), ), ); $query = new WP_Query($args); $items = $query->get_posts(); 

The solution was to use query_post($args) instead of WP_query->get_posts()

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

Can you tell me where i'm wrong ?