Profile page's title/heading does not fulfill hook_user_format_name_alter

I am not sure if this is an actual bug or not, so I assigned category "Support Request"

I've implemented hook_user_format_name_alter in a custom module, and it works perfectly. The newly formatted username that I have defined is being used instead of the default username value everywhere in the application. Everywhere, that is, except the heading on the user profile pages.

EXAMPLE:
Default Username: "123456"
Custom Username Defined by Hook: "customuser"

"customuser" is displaying for my user in the following places:

• all views that include my user record (example, /admin/people and /admin/content),
• the administrative toolbar
• the raw url for my user profile, which shows "/user/customuser" instead of "/user/123456"
• the heading on my profile's contact tab, which shows "Contact customuser" instead of "Contact 123456"

So, I'm wondering why the title on the main profile page is not affected by hook__user_format_name_alter, especially when the title on the contact page does fulfill the hook's definition. I tried setting up the following RouteSubscriber to override the title on the profile page, but doing so broke something and gave me an "Access Denied" message when trying to access my own profile page (even when testing this out on the admin account):

/**
 * @file
 * Contains \Drupal\MYMODULE\Routing\RouteSubscriber.
 */

// see: https://www.drupal.org/node/2187643

namespace Drupal\MYMODULE\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Listens to the dynamic route events.
 */
class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  public function alterRoutes(RouteCollection $collection) {
    //Right now, this breaks the user's home page.
    if ($route = $collection->get('entity.user.canonical')) {
      $route->setDefaults(['_title', 'CUSTOM PROFILE TITLE']);
    }
  }

}

Thanks in advance for any assistance.