OnComplete callback registered via SseEventSource.register() is not called

SseEventSource msgEventSource = SseEventSource.target(target).build();
        try (SseEventSource eventSource = msgEventSource) {
            eventSource.register(event -> {
                // actions on event, this is called
            }, ex -> {
                // actions on error, this is called
            }, () -> {
                completed.incrementAndGet(); // This is never called
            });
            eventSource.open();
        }