Issues with using the 'where' field in Mixpanel export API causing error 400

$client = new \GuzzleHttp\Client();

$projectId  = 3334207;
$fromDate   = '2025-12-28';
$toDate     = '2025-12-28';
$eventName  = 'checkout';
$distinctId = '4612';

$where = urlencode('properties["distinct_id"] == "3147"');

$response = $client->request('GET', 'https://data-eu.mixpanel.com/api/2.0/export', [
    'http_errors' => false, // IMPORTANT: lets you read the body on 400
    'auth' => [$username, $secret], // better than hardcoding a base64 header
    'headers' => [
        'accept' => 'text/plain',
    ],
    'query' => [
        'project_id' => $projectId,
        'from_date'  => $fromDate,
        'to_date'    => $toDate,
        'limit'      => 1,
        'event'      => json_encode([$eventName]),
        'where'      => $where,
    ],
]);

echo "HTTP: " . $response->getStatusCode() . "\n";
echo (string) $response->getBody();