Alexa can't discover device if interface is changed from Alexa.PowerController to Alexa.DoorbellEventSource

I'm quite new to Alexa Smarthome Skills so I followed the steps listed in the following example and was able to get Alexa to discover the "Sample Switch" device:

https://github.com/alexa-samples/skill-sample-nodejs-smarthome-switch/blob/master/instructions/README.md

I then followed the steps listed in the following tutorial to create a Doorbell Smarthome device:

https://www.webgate.biz/aktuelles/blog/doorbell-event-source-for-alexa

But Alexa couldn't discover the device.

What I essentially did was, to modify the following code section of the lambda function:

    if (namespace.toLowerCase() === 'alexa.discovery') {
        let adr = new AlexaResponse({"namespace": "Alexa.Discovery", "name": "Discover.Response"});
        let capability_alexa = adr.createPayloadEndpointCapability();
            let capability_alexa_powercontroller = adr.createPayloadEndpointCapability({"interface": "Alexa.PowerController", "supported": [{"name": "powerState"}]});
            adr.addPayloadEndpoint({"friendlyName": "Sample Switch", "endpointId": "sample-switch-01", "capabilities": [capability_alexa, capability_alexa_powercontroller]});
            return sendResponse(adr.get());
    }

...to this:

    if (namespace.toLowerCase() === 'alexa.discovery') {
        let adr = new AlexaResponse({"namespace": "Alexa.Discovery", "name": "Discover.Response"});
        let capability_alexa = adr.createPayloadEndpointCapability();
        let capability_alexa_doorbell = adr.createPayloadEndpointCapability({"interface": "Alexa.DoorbellEventSource"});
        adr.addPayloadEndpoint({"friendlyName": "Front door", "endpointId": "doorbell-01", "displayCategories": [ "DOORBELL" ], "capabilities": [capability_alexa, capability_alexa_doorbell]});
        return sendResponse(adr.get());
    }

I use the following Test to test the lambda function:

    {
      "directive": {
        "header": {
          "namespace": "Alexa.Discovery",
          "name": "Discover",
          "payloadVersion": "3",
          "messageId": "1bd5d003-31b9-476f-ad03-71d471922820"
        },
        "payload": {
          "scope": {
            "type": "BearerToken",
            "token": "access-token-from-skill"
          }
        }
      }
    }

...and get the following response (for the doorbell), which seems fine:

    Response
    {
      "event": {
        "header": {
          "namespace": "Alexa.Discovery",
          "name": "Discover.Response",
          "messageId": "29ffa439-cc19-47f6-8996-7da21bb79420",
          "payloadVersion": "3"
        },
        "payload": {
          "endpoints": [
            {
              "capabilities": [
                {
                  "type": "AlexaInterface",
                  "interface": "Alexa",
                  "version": "3"
                },
                {
                  "type": "AlexaInterface",
                  "interface": "Alexa.DoorbellEventSource",
                  "version": "3"
                }
              ],
              "description": "Sample Endpoint Description",
              "displayCategories": [
                "DOORBELL"
              ],
              "endpointId": "doorbell-01",
              "friendlyName": "Front door",
              "manufacturerName": "Sample Manufacturer"
            }
          ]
        }
      }
    }

I figured out that the problem was a missing "proactivelyReported": true capability for the DoorBellEventSource interface.

The correct response should've looked like this:

{
  "event": {
    "header": {
      "namespace": "Alexa.Discovery",
      "name": "Discover.Response",
      "messageId": "9b3c332e-73ca-42e8-9dbf-7f313ac5fb02",
      "payloadVersion": "3"
    },
    "payload": {
      "endpoints": [
        {
          "capabilities": [
            {
              "type": "AlexaInterface",
              "interface": "Alexa",
              "version": "3",
              "proactivelyReported": false
            },
            {
              "type": "AlexaInterface",
              "interface": "Alexa.DoorbellEventSource",
              "version": "3",
              "proactivelyReported": true
            }
          ],
          "description": "Sample Endpoint Description",
          "displayCategories": [
            "DOORBELL"
          ],
          "endpointId": "sample-doorbell-01",
          "friendlyName": "Sample DoorbellEventSource",
          "manufacturerName": "Sample Manufacturer"
        }
      ]
    }
  }
}