TASK_ASSIGNMENT.CREATED webhook v2 event not firing

I'm testing the full set of Webhook event types that comprise the v2 API. Everything has worked consistently so far. I can track both file and folder events and receive notifications at the callback URL I provide. 

However,  TASK_ASIGNMENT.CREATED and TASK_ASSIGNMENT.UPDATED do not trigger the Webhook callback event, and I am trying to understand if there is anything special about this particular event.

Description of Test:

I am calling the Box REST APIs using an OAuth token granted during an OAuth session. I have removed the token, but otherwise here is the message exchange where I establish a successful subscription on a folder under my control (I created it in Box myself). Notice how everything works as expected, including two test subscriptions that I provide as proof points (COMMENT.CREATED and COMMENT.UPDATED).

{
  "input": {
    "m": {
      "url": "https://api.box.com/2.0",
      "path": "/webhooks",
      "method": "post"
    },
    "b": {
      "target": {
        "id": "85...25",
        "type": "folder"
      },
      "address": "",
      "triggers": [
        "COMMENT.CREATED",
        "COMMENT.UPDATED",
        "TASK_ASSIGNMENT.CREATED",
        "TASK_ASSIGNMENT.UPDATED"
      ]
    }
  },
  "output": {
    "id": "21...06",
    "type": "webhook",
    "target": {
      "id": "85...25",
      "type": "folder"
    },
    "created_by": {
      "type": "user",
      "id": "90...88",
      "name": "Edapape Beach",
      "login": "***email address removed for privacy***"
    },
    "created_at": "2019-08-21T10:10:53-07:00",
    "address": "",
    "triggers": [
      "COMMENT.CREATED",
      "COMMENT.UPDATED",
      "TASK_ASSIGNMENT.CREATED",
      "TASK_ASSIGNMENT.UPDATED"
    ],
    "#code#": 201
  }
}

You can see from the BOX response above (STATUS CODE 201) that all four subscriptions succeeded. In order to test, I uploaded a file to the target folder and then added a comment to the file using the Box Web UI. As expected, my registered URL received the COMMENT.CREATED event shown here. 

{
  "output": {
    "type": "webhook_event",
    "id": "b9...23",
    "created_at": "2019-08-21T10:17:52-07:00",
    "trigger": "COMMENT.CREATED",
    "webhook": {
      "id": "21...06",
      "type": "webhook"
    },
    "created_by": {
      "type": "user",
      "id": "90...88",
      "name": "Edapape Beach",
      "login": "***email address removed for privacy***"
    },
    "source": {
      "id": "27...18",
      "type": "comment",
      "is_reply_comment": false,
      "message": "This is a comment",
      "created_by": {
        "type": "user",
        "id": "90...88",
        "name": "Edapape Beach",
        "login": "***email address removed for privacy***"
      },
      "created_at": "2019-08-21T10:17:52-07:00",
      "item": {
        "id": "51...51",
        "type": "file"
      },
      "modified_at": "2019-08-21T10:17:52-07:00"
    },
    "additional_info": []
  },
  "operation_output": {
    "ack_response": "{\"status\":\"success\"}"
  }
}

I tested the COMMENT.UPDATED event and it works fine as well. However, if I add a task assignment using either the Box Web UI or using the Box APIs (Create Task API followed by Create Task Assignment API), neither triggers the event.

I ran an additional test, suspecting that perhaps there was a documentation error and instead subscribed to a file object instead of a folder and ran the same set of tests. But this had similar results, where only the COMMENT events were triggered and not TASK_ASSIGNMENT events.

I have confirmed that the OAuth scopes are correct and have selected both root_readwrite and manage_webhook scopes. I assume these are correct; otherwise, the original subscription would have failed and I wouldn't receive COMMENT events. As a final note, I am only assigning tasks to myself on files and folders I created and own, using OAuth credentials bound to a Box App I own. Every asset is under my control. Any insight would be appreciated. Thx.