How to watch for changes to specific fields in MongoDB change stream with Spring Data Mongodb

Question

According to the documentation of the mongodb changestream, I can watch for updates on specific fields on the collecttion:

const changeStream = collection.watch(
[{
  $match: {
    $and: [
      { "updateDescription.updatedFields.a": { $exists: true } },
      { operationType: "update" }
    ]
  }
}],
{
  fullDocument: "updateLookup"
}
);

I would like to know, how to achieve the same thing with spring data mongodb reactive.

When I build a change stream with

Flux<ChangeStreamEvent<TestObject>> changeStream = 
mongoTemplate.changeStream(
        newAggregation(
            match(
                where("operationType").is("update")
                    .and("updateDescription").exists(true)
            )
        ),
        TestObject.class,
        ChangeStreamOptions.empty(),
        "testObject"
    );

I get all updates. A log of one of these change events looks like:

ChangeStreamEvent {
raw=ChangeStreamDocument{
    resumeToken={ "_data" : { "$binary" : "glsU9sYAAAATRmRfaWQAZFsU9sayZFockO0phgBaEATGaP42X4VI+4SrTyscwRtsBA==", "$type" : "00" } },
    namespace=test.testObject, 
    fullDocument=Document{{_id=5b14f6c6b2645a1c90ed2986, a=a, b=b, _class=com.example.demo.TestObject}}, 
    documentKey={ "_id" : { "$oid" : "5b14f6c6b2645a1c90ed2986" } }, 
    clusterTime=null, 
    operationType=OperationType{value='update'}, 
    updateDescription=UpdateDescription{removedFields=[], updatedFields={ "b" : "b" }}}, 
    targetType=class com.example.demo.TestObject
    }

However, when I change the filter to

newAggregation(
            match(
                where("operationType").is("update")
                    .and("updateDescription.updatedFields").exists(true)
            )
        )

or

newAggregation(
            match(
                where("operationType").is("update")
                    .and("updateDescription.updatedFields.b").exists(true)
            )
        )

I get not change events. So here is the question: how can I define the Criteria to match updates on updatedField.b?

A sample Document from the collection is:

{
  "_id" : ObjectId("5b14f049b2645a4a24b33df1"),
  "a" : "va",
  "b" : "vb",
  "_class" : "com.example.demo.TestObject"
}

The example with the first "working" filter can be downloaded at: my test github repo