yii2: captcha is incorrect

messageBoard model

<?php

namespace frontend\models;

use Yii;

/**
 * This is the model class for table "message_board".
 *
 * @property integer $id
 * @property string $firstname
 * @property string $lastname
 * @property string $telephone
 * @property string $email
 * @property string $content
 * @property string $date_added
 */
class MessageBoard extends \yii\db\ActiveRecord
{
    public $verifyCode;

    /**
     * @inheritdoc
     */
    public static function tableName()
    {
        return 'message_board';
    }

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['firstname', 'lastname', 'email', 'content'], 'required'],
            //[['date_added'], 'safe'],
            [['firstname', 'lastname', 'email', 'content'], 'string', 'max' => 255],
            [['telephone'], 'string', 'max' => 10],
            // verifyCode needs to be entered correctly
            ['verifyCode', 'captcha']
        ];
    }

    /**
     * @inheritdoc
     */
    public function attributeLabels()
    {
        return [
            'id' => 'ID',
            'firstname' => 'Firstname',
            'lastname' => 'Lastname',
            'telephone' => 'Telephone',
            'email' => 'Email',
            'content' => 'Content',
            'date_added' => 'Date Added',
            'verifyCode' => 'Verification Code',
        ];
    }

    //get all message in the board
    public function getMessages(){
        return static::find()->all();
    }

}

sitecontroller, the problem is on the actionContact

<?php
namespace frontend\controllers;

use Yii;
use common\models\LoginForm;
use frontend\models\PasswordResetRequestForm;
use frontend\models\ResetPasswordForm;
use frontend\models\SignupForm;
use frontend\models\ContactForm;
use frontend\models\MessageBoard;
use yii\base\InvalidParamException;
use yii\web\BadRequestHttpException;
use yii\web\Controller;
use yii\filters\VerbFilter;
use yii\filters\AccessControl;

/**
 * Site controller
 */
class SiteController extends Controller
{
    /**
     * @inheritdoc
     */
    public function behaviors()
    {
        return [
            'access' => [
                'class' => AccessControl::className(),
                'only' => ['logout', 'signup'],
                'rules' => [
                    [
                        'actions' => ['signup'],
                        'allow' => true,
                        'roles' => ['?'],
                    ],
                    [
                        'actions' => ['logout'],
                        'allow' => true,
                        'roles' => ['@'],
                    ],
                ],
            ],
            'verbs' => [
                'class' => VerbFilter::className(),
                'actions' => [
                    'logout' => ['post'],
                ],
            ],
        ];
    }

    /**
     * @inheritdoc
     */
    public function actions()
    {
        return [
            'error' => [
                'class' => 'yii\web\ErrorAction',
            ],
            'captcha' => [
                'class' => 'yii\captcha\CaptchaAction',
                // 'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
            ],
        ];
    }

    public function actionIndex()
    {
        return $this->render('index');
    }

    public function actionContact()
    {
        $model = new MessageBoard();

        if ($model->load(Yii::$app->request->post()) && $model->validate()) {
            $model->save();

            return $this->refresh();
        } else {
            return $this->render('contact', [
                'model' => $model,
            ]);
        }
    }
}

captcha file: vendor\yiii2soft\yii2\captcha\CaptchaAction.php

when the code run to $model->validate(), the captcha verify return true

but after that, in the process of $model->save(), the captcha is changed. and validate return false.

then I cant save data because the captcha is always incorrect