WordPress meta_query syntax issue

Question

Can anyone work out why the following syntax does work:

$results_args = array(
    'post_type' => 'event',
    'meta_key' => 'start_date',
    'meta_value' => '20161227',
    'posts_per_page' => 30
);
$results = new WP_Query($results_args);

but the following syntax does not?

$results_args = array(
    'post_type' => 'event',
    'meta_query' => array(
        array(
            'key'     => 'start_date',
            'value'   => '20161227',
            'compare' => '=',
        ),
    ),
);
$results = new WP_Query($results_args);

The latter code returns all posts where post_type = 'event'.

Docs are at https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters.

I'm looking to add multiple meta queries in, so I need the latter syntax to work.

Any help would be much appreciated.

Thanks,

Jamie