Update Custom Post Meta Even if Draft

get_post_meta() without the third parameter will return an empty array if the key isn’t found, so it looks like your check on $remaining_issues will fail every time.

Try

$remaining_issues = get_post_meta($post_id, '_remaining_issues',true);

if ($remaining_issues === false) {  // we only want to update to the default 6 if this has not already been set
              update_post_meta($post_id, '_remaining_issues', '6');
}