CDC with Dataflow into BigQuery: Storage Write API doesn't respect Primary Key

[CBOS]

I have a BigQuery table created using the following DDL:

CREATE TABLE mytable AS
(
  id STRING,
  source STRING,
  PRIMARY KEY (id) NOT ENFORCED
);

As you can see, id is set as the table Primary Key. My Beam pipeline is then defined as follows:

def process_message(message):  
    data = json.loads(message.decode("utf-8"))
    if data == {}:
        print(f"Running DELETE operation on row {this_message['Key']}")
        data['_CHANGE_TYPE'] = 'DELETE'
    else:   
        print(f"Running UPSERT operation on row {this_message['Key']}")
        data['_CHANGE_TYPE'] = 'UPSERT'
    data['_CHANGE_SEQUENCE_NUMBER'] = str(struct.pack('d', int(round(float(this_message['Value']['updated'])))).hex())
    return [data]


with beam.Pipeline(options=PipelineOptions([
    f"--project={project_id}",
    "--region=europe-west2",
    "--runner=DataflowRunner",
    "--streaming",
    "--temp_location=gs://tmp/cdc",
    "--staging_location=gs://tmp/cdc",
])) as pipeline:
  data = pipeline | 'ReadFromPubSub' >> ReadFromPubSub(subscription=f'projects/{project_id}/subscriptions/{bq_table_name}')

  data | 'ProcessMessages' >> beam.ParDo(process_message) | 'WriteToBigQuery' >> WriteToBigQuery(
    f'{project_id}:{bq_dataset}.{bq_table_name}',
    schema="id STRING,source STRING",
    method=beam.io.WriteToBigQuery.Method.STORAGE_WRITE_API
  )

However, when I query my table in BigQuery after consuming a handful of records, I have hundreds of duplicates of the id field.

How do I get my pipeline to respect the primary key and perform an UPSERT operation, as it should do?