Pipeline change webhook never contains merge request information

Hi! I'm trying to use the webhooks API, and I'm able to receive webhooks, but for any commit that triggers a pipeline, even if it's part of a merge request, the payload always contains merge_request: null.

Example payload:

{
  "object_kind": "pipeline",
  "object_attributes": {
    "id": 181519610,
    "ref": "demo3",
    "tag": false,
    "sha": "031aa33c35850bee69907ede354d80db1d0b71a0",
    "before_sha": "0000000000000000000000000000000000000000",
    "source": "push",
    "status": "success",
    "detailed_status": "passed",
    "stages": [
      "build"
    ],
    "created_at": "2020-08-24 23:25:33 UTC",
    "finished_at": "2020-08-24 23:25:51 UTC",
    "duration": 15,
    "variables": [

    ]
  },
  "merge_request": null,
  ...
}

and here you can see that the pipeline in question (id: 181519610) is indeed part of the merge request.

This is true for both the payload stored in the webhook log inside gitlab itself, as well as the JSON payloads that I can see in the application that receives them.