Firebase Hosting Rewrites have Empty Request Body

So I'm doing rewrites to a custom subdomain using Firebase Hosting as such. The only issue is that the request body is empty when I send a POST to the rewrite.

That being said, if I hit the native Firebase URL it's fine. Any thoughts?

  "rewrites": [
      {
        "source": "/api/webhook",
        "function": "webhook"
      },
      {
        "source": "**",
        "destination": "/index.html"
      }