### 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。

* K - requestId, V - StreamObserver
     */
    private final Map<Long, StreamObserver<DiscoveryRequest>> requestObserverMap = new ConcurrentHashMap<>();
    /**
     * Store Delta-ADS Request Observer ( StreamObserver in Streaming Request )
     * K - requestId, V - StreamObserver
     */
    private final Map<Long, ScheduledFuture<?>> observeScheduledMap = new ConcurrentHashMap<>();
    /**
     * Store CompletableFuture for Request ( used to fetch async result in ResponseObserver )
     * K - requestId, V - CompletableFuture
     */
    private final Map<Long, CompletableFuture<T>> streamResult = new ConcurrentHashMap<>();
    private final ScheduledExecutorService pollingExecutor;
    private final int pollingTimeout;
    protected final static AtomicLong requestId = new AtomicLong(0);
    public AbstractProtocol(XdsChannel xdsChannel, Node node, int pollingPoolSize, int pollingTimeout) {
        this.xdsChannel = xdsChannel;
        this.node = node;
        this.pollingExecutor = new ScheduledThreadPoolExecutor(pollingPoolSize, new NamedThreadFactory("Dubbo-registry-xds"));
        this.pollingTimeout = pollingTimeout;
    }
    /**
     * Abstract method to obtain Type-URL from sub-class
     *
     * @return Type-URL of xDS
     */
    public abstract String getTypeUrl();
    @Override
    public T getResource(Set<String> resourceNames) {