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.
func_details = arn_to_lambda.get(arn)
    if not func_details:
        return not_found_error(arn)
    result = {'Tags': func_details.tags}
    return jsonify(result)
@app.route('/<version>/tags/<arn>', methods=['POST'])
def tag_resource(version, arn):
    data = json.loads(request.data)
    tags = data.get('Tags', {})
    if tags:
        func_details = arn_to_lambda.get(arn)
        if not func_details:
            return not_found_error(arn)
        if func_details:
            func_details.tags.update(tags)
    return jsonify({})
@app.route('/<version>/tags/<arn>', methods=['DELETE'])
def untag_resource(version, arn):
    tag_keys = request.args.getlist('tagKeys')
    func_details = arn_to_lambda.get(arn)
    if not func_details:
        return not_found_error(arn)
    for tag_key in tag_keys:
        func_details.tags.pop(tag_key, None)
    return jsonify({})
def serve(port, quiet=True):
    
    LAMBDA_EXECUTOR.startup()
    generic_proxy.serve_flask_app(app=app, port=port, quiet=quiet)