Почему не работает ajax отправка письма с jQuery 3.1?

Сразу скажу, при подключении jQuery 1.11 - все отлично работает.
С jQuery 3.11 - страница перезагружается и - все. Ошибок в консоле нет.

<form id="request">
							<!-- Hidden Required Fields -->
							<input type="hidden" name="project_name" value="Название сайта">
							<input type="hidden" name="admin_email" value="admin@mail.ru">
							<input type="hidden" name="form_subject" value="Форма заявки">
							<!-- END Hidden Required Fields -->
						<div class="form-group">
							<label for="phone">Телефон</label>
							<input type="text" class="form-control" name="phone" required="required" id="phone" placeholder="Номер телефона">
						</div>
						<div class="form-group">
							<label for="town">Город</label>
							<input type="text" class="form-control" name="town" required="required" id="town" placeholder="Ваш город">
						</div>
						<input type="submit" class="btn btn-warning" value="Отправить заявку" name="submit">
                                  </form>

$(document).ready(function() {
    //E-mail Ajax Send
    $("#request").submit(function() { 
        var th = $(this);
        $.ajax({
            type: "POST",
            url: "mail.php", 
            data: th.serialize()
        }).done(function() {
            alert("Спасибо!");
            setTimeout(function() {
                // Done Functions
                th.trigger("reset");
            }, 1000);
        });
        return false;
    });
});

<?php
$method = $_SERVER['REQUEST_METHOD'];
//Script Foreach
$c = true;
if ( $method === 'POST' ) {
	$project_name = trim($_POST["project_name"]);
	$admin_email  = trim($_POST["admin_email"]);
	$form_subject = trim($_POST["form_subject"]);

	foreach ( $_POST as $key => $value ) {
		if ( $value != "" && $key != "project_name" && $key != "admin_email" && $key != "form_subject" ) {
			$message .= "
			" . ( ($c = !$c) ? '<tr>':'<tr style="background-color: #f8f8f8;">' ) . "
				<td style='padding: 10px; border: #e9e9e9 1px solid;'><b>$key</b></td>
				<td style='padding: 10px; border: #e9e9e9 1px solid;'>$value</td>
			</tr>
			";
		}
	}
} else if ( $method === 'GET' ) {

	$project_name = trim($_GET["project_name"]);
	$admin_email  = trim($_GET["admin_email"]);
	$form_subject = trim($_GET["form_subject"]);

	foreach ( $_GET as $key => $value ) {
		if ( $value != "" && $key != "project_name" && $key != "admin_email" && $key != "form_subject" ) {
			$message .= "
			" . ( ($c = !$c) ? '<tr>':'<tr style="background-color: #f8f8f8;">' ) . "
				<td style='padding: 10px; border: #e9e9e9 1px solid;'><b>$key</b></td>
				<td style='padding: 10px; border: #e9e9e9 1px solid;'>$value</td>
			</tr>
			";
		}
	}
}

$message = "<table style='width: 100%;'>$message</table>";

function adopt($text) {
	return '=?UTF-8?B?'.Base64_encode($text).'?=';
}

$headers = "MIME-Version: 1.0" . PHP_EOL .
"Content-Type: text/html; charset=utf-8" . PHP_EOL .
'From: '.adopt($project_name).' <'.$admin_email.'>' . PHP_EOL .
'Reply-To: '.$admin_email.'' . PHP_EOL;

mail($admin_email, adopt($form_subject), $message, $headers );