Put this code in functions.php file you iframe will work -

function allow_iframes_in_acf($allowed_tags) {
    $allowed_tags['iframe'] = [
        'src'             => true,
        'width'           => true,
        'height'          => true,
        'frameborder'     => true,
        'allowfullscreen' => true,
        'allow'           => true,
    ];
    return $allowed_tags;
}
add_filter('wp_kses_allowed_html', 'allow_iframes_in_acf', 10, 1);