Notifications API doesn't give URL for discussions

https://api.github.com/notifications gives the following thread, which refers a discussion:

{
  "id": "1344611935",
  "unread": true,
  "reason": "subscribed",
  "updated_at": "2020-11-25T03:06:42Z",
  "last_read_at": null,
  "subject": {
    "title": "'brew cask install wireshark' command purges Wireshark v3.4.0.",
    "url": null,
    "latest_comment_url": null,
    "type": "Discussion"
  },
  ...
}

I notice the “url” field in “subject” is null.
For all threads referring a pull request, an issue or a commit, the field contains some valid URL .

Is this a intended behavior?
If so, how I get the URL for a discussion?