### Environment

* Dubbo version: 3.1.0
* Operating System version: xxx
* Java version: 1.8
* Istio version: 1.13.2

### Steps to reproduce this issue

1. 运行Istio+proxyless [demo程序 ](https://github.com/apache/dubbo-samples/tree/master/dubbo-samples-xds )，启动后consumer能正常访问provider。
2. 扩大provider副本数为2， consumer无法感知到新增的provider节点，新增的provider节点也没有收到consumer的请求。
3. 删除provider对应的k8s service（取消服务注册），consumer没有感知到服务变化，仍然可以继续调用provider。
4. 恢复provider service后，重启consumer实例，consumer才可以正常发现2个provider节点。

private final long requestId;
        public ResponseObserver(long requestId) {
            this.requestId = requestId;
        }
        @Override
        public void onNext(DiscoveryResponse value) {
            logger.info("receive notification from xds server, type: " + getTypeUrl() + " requestId: " + requestId);
            T result = decodeDiscoveryResponse(value);
            StreamObserver<DiscoveryRequest> observer = requestObserverMap.get(requestId);
            if (observer == null) {
                return;
            }
            observer.onNext(buildDiscoveryRequest(Collections.emptySet(), value));
            CompletableFuture<T> future = streamResult.get(requestId);
            if (future == null) {
                return;
            }
            future.complete(result);
        }
        @Override
        public void onError(Throwable t) {
            logger.error("xDS Client received error message! detail:", t);
        }
        @Override
        public void onCompleted() {
            
        }
    }
}