Preventing captcha code from being cached by wp rocket plugin

Question

I use Captcha Code plugin

When the wp rocket plugin plugin is active, the same captcha code as before will be displayed every time the page is refreshed, This problem persists until I clear the cache wp rocket, What is the cause of this problem? I think this is the function that generates the captcha code

function ctl_captcha_generate_code()
{
    //Settings: You can customize the captcha here
    $image_width = 120;
    $image_height = 40;

    $characters_on_image = get_option('wpcaptcha_total_no_of_characters');
    if($characters_on_image < 3 || $characters_on_image > 6) $characters_on_image = 6;

    $wpcaptcha_type = get_option('wpcaptcha_type');
    $wpcaptcha_letters = get_option('wpcaptcha_letters');

    $font = dirname(__FILE__) . '/monofont.ttf';

    //The characters that can be used in the CAPTCHA code.
    //avoid confusing characters (l 1 and i for example)
    if(!empty($wpcaptcha_type) && $wpcaptcha_type == 'alphanumeric')
    {
        switch($wpcaptcha_letters)
        {
            case 'capital':
                $possible_letters = '23456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
                break;
            case 'small':
                $possible_letters = '23456789bcdfghjkmnpqrstvwxyz';
                break;
            case 'capitalsmall':
                $possible_letters = '23456789bcdfghjkmnpqrstvwxyzABCEFGHJKMNPRSTVWXYZ';
                break;
            default:
                $possible_letters = '23456789bcdfghjkmnpqrstvwxyz';
                break;
        }
    }
    elseif(!empty($wpcaptcha_type) && $wpcaptcha_type == 'alphabets')
    {
        switch($wpcaptcha_letters)
        {
            case 'capital':
                $possible_letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
                break;
            case 'small':
                $possible_letters = 'bcdfghjkmnpqrstvwxyz';
                break;
            case 'capitalsmall':
                $possible_letters = 'bcdfghjkmnpqrstvwxyzABCEFGHJKMNPRSTVWXYZ';
                break;
            default:
                $possible_letters = 'abcdefghijklmnopqrstuvwxyz';
                break;
        }
    }
    elseif(!empty($wpcaptcha_type) && $wpcaptcha_type == 'numbers')
    {
        $possible_letters = '0123456789';
    }
    else
    {
        $possible_letters = '0123456789';
    }
    $random_dots = 0;
    $random_lines = 20;
    $captcha_text_color="0x142864";
    $captcha_noice_color = "0x142864";

    $code = '';


    $i = 0;
    while ($i < $characters_on_image)
    {
        $code .= substr($possible_letters, mt_rand(0, strlen($possible_letters)-1), 1);
        $i++;
    }


    $font_size = $image_height * 0.75;
    $image = @imagecreate($image_width, $image_height);


    /* setting the background, text and noise colours here */
    $background_color = imagecolorallocate($image, 255, 255, 255);

    $arr_text_color = ctl_captcha_hexrgb($captcha_text_color);
    $text_color = imagecolorallocate($image, $arr_text_color['red'],
        $arr_text_color['green'], $arr_text_color['blue']);

    $arr_noice_color = ctl_captcha_hexrgb($captcha_noice_color);
    $image_noise_color = imagecolorallocate($image, $arr_noice_color['red'],
        $arr_noice_color['green'], $arr_noice_color['blue']);


    /* generating the dots randomly in background */
    for( $i=0; $i<$random_dots; $i++ )
    {
        imagefilledellipse($image, mt_rand(0,$image_width), mt_rand(0,$image_height), 2, 3, $image_noise_color);
    }


    /* generating lines randomly in background of image */
    for( $i=0; $i<$random_lines; $i++ ) {
        imageline($image, mt_rand(0,$image_width), mt_rand(0,$image_height), mt_rand(0,$image_width), mt_rand(0,$image_height), $image_noise_color);
    }


    /* create a text box and add 6 letters code in it */
    $textbox = imagettfbbox($font_size, 0, $font, $code);
    $x = ($image_width - $textbox[4])/2;
    $y = ($image_height - $textbox[5])/2;
    imagettftext($image, $font_size, 0, $x, $y, $text_color, $font , $code);

    ob_start();
    imagejpeg($image);//showing the image
    printf('<img src="data:image/png;base64,%s"/ width="100">', base64_encode(ob_get_clean()));
    imagedestroy($image);//destroying the image instance
    $_SESSION['captcha_code'] = $code;
}

function ctl_captcha_hexrgb ($hexstr)
{
    $int = hexdec($hexstr);

    return array("red" => 0xFF & ($int >> 0x10),
        "green" => 0xFF & ($int >> 0x8),
        "blue" => 0xFF & $int);
}