Order by meta_key in custom post type doesn’t affect the query

I’m trying to query custom posts ordering by a meta_key. But after WP_Query running, there isn’t any change in sql query inside the order clause.

$args = array(
'post_type'   => 'event',
'post_status' => 'publish',

'meta_key' => '_start_eventtimestamp',
'orderby' => 'meta_value_datetime',
'order' => 'ASC',

'meta_query' => array(
    array(
        'key' => '_start_eventtimestamp',
        'value' => date('Ymd'),
        'compare' => '>='
    )
),

'posts_per_page' => '7',
);

$event_query = new WP_Query( $args );

And when I get the generated query I got this:

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID FROM wp_posts
 INNER JOIN wp_postmeta ON ( wp_posts.ID = wp_postmeta.post_id )
 INNER JOIN wp_postmeta AS mt1 ON ( wp_posts.ID = mt1.post_id )
 WHERE 1=1 AND ( wp_postmeta.meta_key = '_start_eventtimestamp'
 AND ( ( mt1.meta_key = '_start_eventtimestamp' AND mt1.meta_value >= '20160930' ) ) )
 AND wp_posts.post_type="event" AND ((wp_posts.post_status="publish"))
 GROUP BY wp_posts.ID ORDER BY wp_posts.menu_order ASC LIMIT 0, 7

The order clause suffered no change.
Is there wrong in my script of order by meta_key?

Please have a look in the WP_QUERY improvement and the WP_QUERY Order & Orderby Parameters.

$args = array( 
         'post_type'   => 'event',
         'post_status' => 'publish',
         'orderby' => array(
           'meta_key' => 'ASC'
          ), 
         'meta_query' => array(
           array(
             'key' => '_start_eventtimestamp',
             'value' => date('Ymd'),
             'compare' => '>='
          ),
       ),
    );

Hope it will work for you!