Get a list of all flags created by the current user's flagged users?

If you want to use the session_id (in case of anonymous users), you should use the session id generated by the flag service in stead of the sessionManager. They are different ;-)

    $flag = $this->flagService->getFlagById('flag_id');
    $user = $this->currentUser;
    $session_id = $this->flagService->getAnonymousSessionId();

    $query = $this->database
      ->select('flagging', 'f')
      ->fields('f', [])
      ->condition('flag_id', $flag->id())
      ->condition('uid', $user->id());

    if ($user->isAnonymous()) {
      $query->condition('session_id', $session_id);
    }
    $result = $query->execute()->fetchAll();