You are sending the ID
of the menu item not the ID
of the page try using url_to_postid()
like this:
<?php
$array_menu = wp_get_nav_menu_items('header-menu');
$menu = array();
foreach ($array_menu as $m) {
if (empty($m->menu_item_parent)) {
?>
<h4><a href="https://wordpress.stackexchange.com/questions/257249/<?php echo $m->url;?>"><?php echo $m->title;?></a></h4>
<p>
<?php global $wp_query;
$pageID = url_to_postid( $m->url );
echo get_post_meta($pageID, 'meta_box_subtitle', true);
?>
</p>
<?php
}
}
?>