@sabersma 其实也就是setFlash()参数$removeAfterAccess = true (delete after access) or false (delete until the next call) 两种情况下的自动销毁没有生效，但是其他方式的显式删除flash是可以的。（第四个case == 第一个case, 忽略它吧)

是别的地方有什么需要配置吗？

@sabersma 额，输出部分格式贴乱了，，凑合着看吧

@500miles
额，不好意思我记错了，getFlash()时置delete=true 或是调用$session->removeFlash()都是有效的。但是除此之外的自动销毁还是无效。刚才写了下测试:
actions:

public function actionTestFlash() {
    Yii::$app->session->setFlash('delete when access', 'hello world', true);
    Yii::$app->session->setFlash('delete when next call', 'hello world', false);
    Yii::$app->session->setFlash('set delete when getFlash', 'hello world');
    Yii::$app->session->setFlash('set nothing, wait for auto delete in the next call', 'hello world');
}

public function actionNewCall1() {
    return $this->render('new_call1');
}

public function actionNewCall2() {
    return $this->render('new_call2');
}

new_call1.php:

$session = Yii::$app->session;
var_dump('delete when access' . ' - ' . $session->getFlash('delete when access'));
var_dump('delete when next call' . ' - ' . $session->getFlash('delete when next call'));
var_dump('set delete when getFlash' . ' - ' . $session->getFlash('set delete when getFlash', null, true));
var_dump('set nothing, wait for auto delete in the next call' . ' - ' . $session->getFlash('set nothing, wait for auto delete in the next call'));
var_dump($_SESSION);

new_call2.php:

$session = Yii::$app->session;
$flashes = $session->getAllFlashes();

var_dump($flashes);
var_dump($_SESSION);
$session->removeAllFlashes();

new-call1的输出：

string(32) "delete when access - hello world"
string(35) "delete when next call - hello world"
string(38) "set delete when getFlash - hello world"
string(64) "set nothing, wait for auto delete in the next call - hello world"
array(4) {
  ["delete when access"]=>
  string(11) "hello world"
  ["__flash"]=>
  array(3) {
    ["delete when access"]=>
    int(1)
    ["delete when next call"]=>
    int(0)
    ["set nothing, wait for auto delete in the next call"]=>
    int(1)
  }
  ["delete when next call"]=>
  string(11) "hello world"
  ["set nothing, wait for auto delete in the next call"]=>
  string(11) "hello world"
}

new_call2输出：

array(3) {
  ["delete when access"]=>
  string(11) "hello world"
  ["delete when next call"]=>
  string(11) "hello world"
  ["set nothing, wait for auto delete in the next call"]=>
  string(11) "hello world"
}
array(4) {
  ["delete when access"]=>
  string(11) "hello world"
  ["__flash"]=>
  array(3) {
    ["delete when access"]=>
    int(1)
    ["delete when next call"]=>
    int(0)
    ["set nothing, wait for auto delete in the next call"]=>
    int(1)
  }
  ["delete when next call"]=>
  string(11) "hello world"
  ["set nothing, wait for auto delete in the next call"]=>
  string(11) "hello world"
}

除了在getFlash()时设置delete=true的那次之外其余三次均没有正常销毁，当然，removeAllFlashes()之后整个世界都清净了。

这个我是知道的，补充一下，是在不加载debug module的情况下。getFlash()且非同一次请求，没有销毁这条信息，
即使是在getFlash()中将参数delete显示置为true也没用。

你的$identity 的类需要继承一下IdentityInterface接口