Captcha code changing due to 'unique' validator

I have a registration form that has a user name and a captcha field (along with other fields). The user name field has ‘unique’ ajax validation enabled, and i have code in the site controller to handle the return of $errors. The problem is that it always returns an error stating that the captcha codes don’t match. What appears to be happening is that when ajax triggers upon leaving the user name field (or submitting the form), the captcha value in $_SESSION[’__captcha/site/captcha’] gets changed to a new captcha code before the validation is performed, causing the validation to fail.
model:
public $verifyCode; // for running captcha
rules:
[‘verifyCode’, ‘captcha’,‘message’=> ‘The verification code is incorrect, click on the image if you need a new code.’],
[[‘user_name’, ‘last_name’], ‘string’, ‘max’ => 15],
[[‘user_name’], ‘unique’,‘message’=>‘This Username has been taken, try a different one.’],

View:

<?= $form->field($memberModel, 'verifyCode')->widget(Captcha::className(), [ //'template' => '
{image}
{input}
', 'options'=>['style'=>'width:60%'], ])->label(false) ?>
        <?= $form->field($memberModel, 'user_name', ['enableAjaxValidation' => true])->textInput(['style'=>'width:50%']) ?>

Controller:
public function actionRegistration_form(){
$memberModel = new Member();
if (Yii::$app->request->isAjax && $memberModel->load(Yii::$app->request->post())) {
Yii::$app->response->format = Response::FORMAT_JSON;
return ActiveForm::validate($memberModel);
}… rest of method

Hope it is something stupid I’m doing… anyone have any ideas?