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.
data = json.loads(to_str(request.data))
    result = set_function_code(data, function)
    arn = func_arn(function)
    func_details = arn_to_lambda.get(arn)
    result.update(format_func_details(func_details))
    if isinstance(result, Response):
        return result
    return jsonify(result or {})
@app.route('%s/functions/<function>/code' % PATH_ROOT, methods=['GET'])
def get_function_code(function):
    """ Get the code of an existing function
        ---
        operationId: 'getFunctionCode'
        parameters:
    """
    arn = func_arn(function)
    lambda_cwd = arn_to_lambda[arn].cwd
    tmp_file = '%s/%s' % (lambda_cwd, LAMBDA_ZIP_FILE_NAME)
    return Response(load_file(tmp_file, mode='rb'),
            mimetype='application/zip',
            headers={'Content-Disposition': 'attachment; filename=lambda_archive.zip'})
@app.route('%s/functions/<function>/configuration' % PATH_ROOT, methods=['GET'])
def get_function_configuration(function):
    """ Get the configuration of an existing function
        ---
        operationId: 'getFunctionConfiguration'
        parameters:
    """
    arn = func_arn(function)
    lambda_details = arn_to_lambda.get(arn)