This happens when running //test/common/integration:rtds_integration_test and //test/common/integration:sds_integration_test.

The cause seems to be the initial config fetch timer expiring after 15s: https://github.com/envoyproxy/envoy/blob/3059a76977311b43dc993720f7fda49c8e4dfbee/source/common/config/grpc_subscription_impl.cc#L35. This seems to unblock the FakeUpstream from sending a xDS response, allowing the gRPC stream to proceed and the response is received in the Envoy engine.