May be a duplicate of https://github.com/localstack/localstack/issues/988 but due to the age I will re raise.

I'm having issues when I throw an exception in a lambda - it isn't appearing on my dead letter queue. Is there something I am missing, I would like an sqs event exception to appear on the poison queue when an exception happens.

Below is the basic config:
 ````
    sqsclient.create_queue(QueueName = 'test-queue')
    sqsclient.create_queue(QueueName = 'test-queue-poison')

    lambda_client.create_function(
            FunctionName='MyFunction',
            Runtime='dotnetcore2.1',
            Role='role',
            Handler='Handler',
            Code=code,
	    DeadLetterConfig={'TargetArn': 'test-queue-poison-arn'})

     lambda_client.create_event_source_mapping(
		EventSourceArn='test-queue-arn',
		FunctionName='MyFunction',
		Enabled=True,
		BatchSize=100,
		StartingPosition='LATEST')
````

So when I raise a new (.net)exception within the lambda function no message is going to the dead letter queue. I can confirm that all is attached together OK as all other scenarios work, and the above code is pseudo-ish.
""" Create new event source mapping
        ---
        operationId: 'createEventSourceMapping'
        parameters:
            - name: 'request'
              in: body
    """
    data = json.loads(to_str(request.data))
    mapping = add_event_source(data['FunctionName'], data['EventSourceArn'], data.get('Enabled'))
    return jsonify(mapping)
@app.route('%s/event-source-mappings/<mapping_uuid>' % PATH_ROOT, methods=['PUT'])
def update_event_source_mapping(mapping_uuid):
    """ Update an existing event source mapping
        ---
        operationId: 'updateEventSourceMapping'
        parameters:
            - name: 'request'
              in: body
    """
    data = json.loads(request.data)
    if not mapping_uuid:
        return jsonify({})
    function_name = data.get('FunctionName') or ''
    enabled = data.get('Enabled', True)
    batch_size = data.get('BatchSize') or 100
    mapping = update_event_source(mapping_uuid, function_name, enabled, batch_size)
    return jsonify(mapping)
@app.route('%s/event-source-mappings/<mapping_uuid>' % PATH_ROOT, methods=['DELETE'])
def delete_event_source_mapping(mapping_uuid):