This route can only be accessed by anonymous users

Why are you making this call as authenticated if I may ask?

The same is true for core's /user/register route:

user.register:
  path: '/user/register'
  defaults:
    _entity_form: 'user.register'
    _title: 'Create new account'
  requirements:
    _access_user_register: 'TRUE'

  access_check.user.register:
    class: Drupal\user\Access\RegisterAccessCheck
    tags:
      - { name: access_check, applies_to: _access_user_register }

class RegisterAccessCheck implements AccessInterface {
  public function access(AccountInterface $account) {
    $user_settings = \Drupal::config('user.settings');
    return AccessResult::allowedIf($account->isAnonymous() && $user_settings->get('register') != UserInterface::REGISTER_ADMINISTRATORS_ONLY)->addCacheableDependency($user_settings);
  }
}

This is intentional.