GraphQL response not aligned with public schema (CROSS_REFERENCED_EVENT)

Here is a reproducer query:

POST https://api.github.com/graphql
User-Agent: change-metrics/lentille-morpheus
Authorization: token <the-token>
{
  "variables": {
    "search": "repo:elastic/elasticsearch updated:>=2021-01-01 linked:pr cancellable in:title"
  },
  "operationName": "GetLinkedIssues",
  "query": "query GetLinkedIssues ($search: String!) {\n search(query: $search, type: ISSUE, first: 25) {nodes { ... on Issue { id url timelineItems(first: 100, itemTypes: [CROSS_REFERENCED_EVENT]) { nodes { ... on CrossReferencedEvent { source { ... on PullRequest {url id } } } }}}}}}"
}

Here is the response:

{
  "data": {
    "search": {
      "nodes": [
        {
          "id": "MDU6SXNzdWU4ODYzOTc2MTY=",
          "url": "https://github.com/elastic/elasticsearch/issues/72907",
          "timelineItems": {
            "nodes": [
              {
                "source": {
                }
              },
              {
                "source": {
                }
              },
              {
                "source": {
                  "url": "https://github.com/elastic/elasticsearch/pull/72931",
                  "id": "MDExOlB1bGxSZXF1ZXN0NjQwNjQzMzEz"
                }
              }
            ]
          }
        }
      ]
    }
  }
}

As you can see the “source” field of “CrossReferencedEvent” object should contain a PullRequest or an Issue object but for some issues (like the one above) it is simply empty.

source is a “ReferencedSubject!” object which is either an Issue or a PullRequest. So according to Public schema - GitHub Docs I do not expected an empty field.

Either I misunderstand the schema or the schema is wrong regarding the “type CrossReferencedEvent”.