Traces not sampled with Istio and Sleuth - istio

Question

I am using Spring Boot 2 Microservices with Spring Cloud Sleuth with the Dependency Management and Spring Cloud Version Greenwich.SR2.
My service is running in an Istio service mesh.
Sample policy of istio is set to 100 (pilot.traceSampling: 100.0).
To use distributed tracing in the mesh, the applications needs to forward HTTP headers like the X-B3-TraceId and X-B3-SpanID. This is achieved by simply adding Sleuth. All my HTTP request are are traced correctly. The sidecar proxies of Istio (Envoy) send the traces to the Jaeger backend.
Sleuth is also supposed to work with Spring WebSocket. But my incoming websocket requests do not get any trace or span id by sleuth; Logs look like [-,,,].
1. Question: Why is Sleuth not working for websocket?
My WS-Config:
#Configuration
#EnableWebSocket
public class WsConfig implements WebSocketConfigurer {
#Autowired
WebSocketHandler webSocketHandler;
#Override
public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
DefaultHandshakeHandler handshakeHandler = new DefaultHandshakeHandler();
handshakeHandler.setSupportedProtocols(HANDSHAKE_PROTOCOL);
registry.addHandler(webSocketHandler, WS_HANDLER_PATH + WILDCARD)
.setAllowedOrigins("*")
.setHandshakeHandler(handshakeHandler);
}
}
My clients are able to connect to my Service via Websocket. I am implementing WebSocketHandler interface to handle WS messages.
To achieve that my WS connections are logged by Sleuth, I annotate the method that handles my connection with #NewSpan:
#Override
#NewSpan
public void handleMessage(WebSocketSession session, WebSocketMessage<?> message) {
//doWork and call other services via HTTP
}
With this, Sleuth creates trace and spanId and also propagates them to the other Services, which are called via the restTemplate in this method. But HTTP calls are not send to Jaeger. The x-B3-Sampled Header is always set to 0 by the sidcar.
2 Question: Why are those traces not send to the tracing backend?
Thank you in advance!