[Bug] GraphQL API originalCommit is null

Trying to use the GraphQL API to fetch the originalCommit for a pull request review thread comment.
For some comments, the originalCommit is null.

Seems like a bug. If this is intentional, why is it sometimes null, and how else can I get the originalCommit?

Here’s the query I’m running:

query {
  repository(owner: "nodejs", name: "node") {
    pullRequests(first: 10) {
      nodes {
        number
        reviews(first: 10) {
          nodes {
            databaseId comments(first: 100) {
              nodes {
                databaseId
                author { login }
                commit { oid }
                originalCommit { oid }
              }
            }
          }
        }
        reviewThreads(first: 10) {
          nodes {
            diffSide comments(first: 100) {
              nodes {
                databaseId
                author { login }
                commit { oid }
                originalCommit { oid }
              }
            }
          }
        }
      }
    }
  }
}

You can see in the REST API that the original_commit_id isn’t null:
https://api.github.com/repos/nodejs/node/pulls/comments/21053717

Doing a little more digging, at least some (all?) seem to be ‘not our ref’ issues.
As in, Git doesn’t have the originalCommit (which makes sense).
Presumably the original commit SHA is there, but the graphQL API actually checks to see if git has it, whereas the REST API doesn’t?