Commit is null in close issue event

When I access timeline or events API of issue, the commit field of “closed” event node is null. I check the example of GitHub doc, that seems should not be null.

https://docs.github.com/en/rest/reference/issues#list-timeline-events-for-an-issue--code-samples

[
  {
    "id": 1,
    "node_id": "MDEwOklzc3VlRXZlbnQx",
    "url": "https://api.github.com/repos/octocat/Hello-World/issues/events/1",
    "actor": {
      "login": "octocat",
      "id": 1,
      "node_id": "MDQ6VXNlcjE=",
      "avatar_url": "https://github.com/images/error/octocat_happy.gif",
      "gravatar_id": "",
      "url": "https://api.github.com/users/octocat",
      "html_url": "https://github.com/octocat",
      "followers_url": "https://api.github.com/users/octocat/followers",
      "following_url": "https://api.github.com/users/octocat/following{/other_user}",
      "gists_url": "https://api.github.com/users/octocat/gists{/gist_id}",
      "starred_url": "https://api.github.com/users/octocat/starred{/owner}{/repo}",
      "subscriptions_url": "https://api.github.com/users/octocat/subscriptions",
      "organizations_url": "https://api.github.com/users/octocat/orgs",
      "repos_url": "https://api.github.com/users/octocat/repos",
      "events_url": "https://api.github.com/users/octocat/events{/privacy}",
      "received_events_url": "https://api.github.com/users/octocat/received_events",
      "type": "User",
      "site_admin": false
    },
    "event": "closed",
    "commit_id": "6dcb09b5b57875f334f61aebed695e2e4193db5e",
    "commit_url": "https://api.github.com/repos/octocat/Hello-World/commits/6dcb09b5b57875f334f61aebed695e2e4193db5e",
    "created_at": "2011-04-14T16:00:49Z"
  }
]

but i get from https://api.github.com/repos/confluentinc/ksql/issues/5062/events

{
        "id": 3253869596,
        "node_id": "MDExOkNsb3NlZEV2ZW50MzI1Mzg2OTU5Ng==",
        "url": "https://api.github.com/repos/confluentinc/ksql/issues/events/3253869596",
        "actor": {
            "login": "agavra",
            "id": 3172405,
            "node_id": "MDQ6VXNlcjMxNzI0MDU=",
            "avatar_url": "https://avatars3.githubusercontent.com/u/3172405?v=4",
            "gravatar_id": "",
            "url": "https://api.github.com/users/agavra",
            "html_url": "https://github.com/agavra",
            "followers_url": "https://api.github.com/users/agavra/followers",
            "following_url": "https://api.github.com/users/agavra/following{/other_user}",
            "gists_url": "https://api.github.com/users/agavra/gists{/gist_id}",
            "starred_url": "https://api.github.com/users/agavra/starred{/owner}{/repo}",
            "subscriptions_url": "https://api.github.com/users/agavra/subscriptions",
            "organizations_url": "https://api.github.com/users/agavra/orgs",
            "repos_url": "https://api.github.com/users/agavra/repos",
            "events_url": "https://api.github.com/users/agavra/events{/privacy}",
            "received_events_url": "https://api.github.com/users/agavra/received_events",
            "type": "User",
            "site_admin": false
        },
        "event": "closed",
        "commit_id": null,
        "commit_url": null,
        "created_at": "2020-04-20T22:54:52Z",
        "performed_via_github_app": null
    }

It seems if close by PR. so, commit is null