Originally reported in https://github.com/localstack/localstack/issues/2099, the CreationTime for CF stacks was hardcoded. This was fixed in in LocalStack itself (https://github.com/localstack/localstack/pull/2103), but is now also fixed in moto (https://github.com/spulec/moto/pull/2818).

I don't know what version of moto you're using, and whether it's acceptable to use dev-releases, but if you do: 

moto 1.3.15.dev550 should fix this issue, so the LS fix can be removed if you upgrade :) 
try:
        client = aws_stack.connect_to_service('cloudformation')
        stack = client.describe_stacks(StackName=stack_name)['Stacks']
    except Exception:
        return None
    stack = stack and stack[0]
    if not stack:
        return None
    result = [p['ParameterValue'] for p in stack['Parameters'] if p['ParameterKey'] == parameter]
    return (result or [None])[0]
def update_resource(resource_id, resources, stack_name):
    resource = resources[resource_id]
    resource_type = get_resource_type(resource)
    if resource_type not in UPDATEABLE_RESOURCES:
        LOG.warning('Unable to update resource type "%s", id "%s"' % (resource_type, resource_id))
        return
    LOG.info('Updating resource %s of type %s' % (resource_id, resource_type))
    props = resource['Properties']
    if resource_type == 'Lambda::Function':
        client = aws_stack.connect_to_service('lambda')
        keys = ('FunctionName', 'Role', 'Handler', 'Description', 'Timeout', 'MemorySize', 'Environment', 'Runtime')
        update_props = dict([(k, props[k]) for k in keys if k in props])
        update_props = resolve_refs_recursively(stack_name, update_props, resources)
        if 'Code' in props:
            client.update_function_code(FunctionName=props['FunctionName'], **props['Code'])
        return client.update_function_configuration(**update_props)
    if resource_type == 'ApiGateway::Method':
        client = aws_stack.connect_to_service('apigateway')
        integration = props.get('Integration')