ajax validation marks inputs as invalid despite their value changed

[zuozp8]

What steps will reproduce the problem?

• setup Yii2 project from Yii2 template
• in views/site/contact.php change
  <?php $form = ActiveForm::begin(['id' => 'contact-form']); ?> into
  <?php $form = ActiveForm::begin(['id' => 'contact-form', 'enableAjaxValidation' => true, 'enableClientValidation' => false]); ?>
• in controllers/SiteController.php insert

    if (Yii::$app->request->isAjax && $model->load(Yii::$app->request->post())) {
            sleep(5);
            Yii::$app->response->format = \yii\web\Response::FORMAT_JSON;
            return \yii\bootstrap\ActiveForm::validate($model);
    }

after $model = new ContactForm();

• run ./yii serve
• enter /index.php?r=site%2Fcontact
• quickly fill name and subject fields, then click other field (all must be completed in 5 seconds)

What is the expected result?

Those 2 fields marked as valid

What do you get instead?

Only first field is marked as valid

Additional info

Q	A
Yii version	c586f19 (between 2.0.11.2 and 2.0.12)
PHP version	7.1
Operating system	Ubuntu

I suspect following flow:

• value of email field is changed
• first validation ajax is sent with data like {'name': 'foo', 'subject':''}
• value of subject field is changed
• second validation ajax is sent with data like {'name': 'foo', 'subject':'foo'}
• first validation ajax returns, marks subject field as invalid and non-dirty
• second validation ajax returns, but there are no dirty fields, so it does nothing