Why is WP_Query not displaying expected data?

You must change your date to an ordering like @ Krzysiek Dróżdż said or use meta_type options. I have tested with the string version.

An example of meta_type $args is listed below. See also MySQL time variants

/*
Plugin Name: Meta Filter
Plugin URI: https://wordpress.stackexchange.com/questions/304333
*/

// ?XDEBUG_SESSION_START=304333

function wpse_304333() {
    echo '<p>wpse_304333</p>';

    $args = array(
        'post_type'  => 'my-post-type',
        'meta_key'   => 'my-date-as-string-yyyy-mm-dd',
        'meta_value' => '2018-03-00', // before march 2018
        'meta_compare' => '>',
    );

    $query = new WP_Query( $args );

    $titles="";
    while ( $query->have_posts() ) : $query->the_post();
        $titles .= '<br/>' . get_the_title();
    endwhile;
    echo $titles;
    wp_reset_postdata();
}

add_action( 'admin_notices', 'wpse_304333' );

uses an SQL containing something like

AND ( 
  ( wp_postmeta.meta_key = 'my-date-as-string-yyyy-mm-dd'
    AND wp_postmeta.meta_value < '2018-03-00' )
)

Or use meta_type like

$args = array(
    'post_type'  => 'my-post-type',
    'meta_key'   => 'my-date-key',
    'meta_value' => $date_value
    'meta_compare' => '>',
    'meta_type' => 'DATETIME'
);

which is SQL converted using CAST. You can run the SQL below to check what values are ‘cast’ properly.

SELECT meta_value, CAST(meta_value AS DATETIME)
  FROM wp_postmeta
 WHERE CAST(meta_value AS DATETIME) IS NOT NULL

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