I just tested again and I can create several tasks in a project and I receive a webhook for each, not just the first one.
The only thing I can think of is, are you returning an “OK” (200) response when you receive a webhook, and also are you adding an an “X-Hook-Signature” header in your response with the X-Hook-Signature value that you received in the webhook? I’m not actually sure whether setting that header is necessary or not, but I do it and my webhooks are working properly.