Symfony Guzzle - Post request failing - When I try to debug the request body I get null

I am working on an old project using Symfony 3.4 and Guzzle 5.3.

In my code I have something like that:

$json = $this->serializer->serialize($objectInstance, 'json');

try {
    $response = $this->client->post(
        'documents',
        [
            "body" => $json,
        ]
    );

    dump(json_decode($response->getBody()->getContents()));
    die();
} catch(Throwable $e) {
    /** @var Request $request */
    $request = $e->getRequest();
    dump("Request");
    dump($request->getBody()->getContents());
}

But the output I get is like this:

MyClass.php on line 55:
"Request"

MyClass.php on line 58:
""

So far, I am 100% sure that the $json I use as "body" param is a valid JSON Document and 100% not empty.

Any idea why I could have this behaviour? Maybe it's something I don't see, but I have try a lot and I cannot figure out why it doesn't work.