Customing the contact form - problem : can't add data

Controller

You used correct, no needed any change.

$kirby->email([
    'template' => 'email',
    'from'     => 'contact@lacuverieduchateau.com',
    'replyTo'  => $data['email'],
    'to'       => 'adrien.payet@outlook.com',
    'subject'  => esc($data['name']) . ' a envoyé un message via le site internet',
    'data'     => [
        'tel'    => esc($data['tel']),
        'pers'   => est($data['pers']),
        'text'   => esc($data['text']),
        'sender' => esc($data['name']),
    ]
]);

Template

Correct

<input type="number" id="tel" name="tel" value="<?= $tel ?? '' ?>">

Wrong

<input type="number" id="tel" name="tel" value="<?= $data['tel'] ?? '' ?>">

Because the content of the data index is sent directly to the template: