Hot Chocolate GraphQL batching error when using a stitched schema

Question

I have two 3 Hot Chocolate GraphQL endpoints. Two of these are for separate services; the third endpoint is a "gateway" which uses schema stitching to merge the schemas from the other two endpoints. At the moment I'm just doing a basic merge of the two schemas;

        services
        .AddGraphQLServer()
        .AddRemoteSchema(WellKnownSchemaNames.Applicability)
        .AddRemoteSchema(WellKnownSchemaNames.Modifications);

Within the modification service I've been able to batch two mutations;

mutation a {

  addModification( 
    input: {serialNumber: "abc/1234", owner: "dc78154eac0149958883b344e800fc62", ownerName: "Test", description: "alex 9", title: "alex 9", categories : [1, 2]}) {
    modification {
      id @export(as: "id")
    }
  }
}

mutation b($id:Uuid!) {
  addSomthingElse(id: $id) {
    modification {
      id
      serialNumber
    }
  }
}

And posted this to the service endpoint http://localhost/ModificationService.API/graphql/?batchOperations=[a,b] and this has worked returning a HTTP 200 response and the body content;

---
Content-Type: application/json; charset=utf-8
Content-Length: 87

{"data":{"addModification":{"modification":{"id":"01df5762244a4fe7a55a8b7d1166ddc3"}}}}

---
Content-Type: application/json; charset=utf-8
Content-Length: 113

{"data":{"addSomthingElse":{"modification":{"id":"01df5762244a4fe7a55a8b7d1166ddc3","serialNumber":"abc/1234"}}}}

-----

If I try posting the same two mutations to the gateway url (http://localhost/Gateway.API/graphql/?batchOperations=[a,b]), I get the error;

{
    "errors": [
        {
            "message": "Either the parameter query or the parameter id has to be set.",
            "extensions": {
                "code": "HC0013"
            }
        }
    ]
}

Does anyone have any suggestions what I've done wrong?