Serverless product event onContactDelete not triggering

Hello,

we have a problem with a serverless product event.
The event onContactDelete is not triggered in our sandbox environment.

I deployed the custom app in the sandbox environment for testing and also tested deleting contacts via Freshesk API with Postman.
I tested soft delete as well as hard delete. The contact is deleted, but the event is not triggered.
Other events like onContactCreate are working properly though.
I can see the log statement of my onContactCreateCallback in the admin panel (manage apps / custom apps / settings / view logs), but there is no output from onContactDeleteCallback. Therefore I assume the onContactDelete event is not triggered.
But in the event simulation at http://localhost:10001/web/test onContactDelete is acutally working!

manifest.json

{
  "platform-version": "2.3",
  "product": {
    "freshdesk": {
      "events": {
        "onAppInstall": {
          "handler": "onAppInstallCallback"
        },
        "onContactDelete": {
          "handler": "onContactDeleteCallback"
        },
        "onContactCreate": {
          "handler": "onContactCreateCallback"
        }
      },
      "requests": {
        "getContact": {},
        "updateContact": {}
      }
    }
  },
  "engines": {
    "node": "18.18.2",
    "fdk": "9.0.8"
  }
}

server.js

exports = {
  onAppInstallCallback: function(payload) {
    console.log("Logging arguments from onAppInstallevent: " + JSON.stringify(payload));
    renderData();
  },
  onContactDeleteCallback: function() {
    console.log("onContactDeleteCallback");
  },
  onContactCreateCallback: function() {
    console.log("onContactCreateCallback");
  }
};

Could you please help me finding the problem?
Thanks in advance!