RxJava: 2.x: Single.flatMapObservable does not respect observeOn

RxJava version 2.1.2.

The following code:

Single.just("Test")
      .subscribeOn(Schedulers.computation())
      .flatMapObservable(
            s -> {
                System.out.println("1: " + Thread.currentThread());

                return Observable.just(1)
                      .observeOn(Schedulers.io())
                      //.doOnNext(o -> System.out.println("2: " + Thread.currentThread()))
                 ;
            }
      )
      .subscribe(o -> {
          System.out.println("3: " + Thread.currentThread());
      });

…produces the following output:

1: Thread[RxComputationThreadPool-1,5,main]
3: Thread[RxComputationThreadPool-1,5,main]

Expected output here is:

1: Thread[RxComputationThreadPool-1,5,main]
3: Thread[RxCachedThreadScheduler-1,5,main]

However, when the line containing doOnNext is uncommented, this is the output:

1: Thread[RxComputationThreadPool-1,5,main]
2: Thread[RxCachedThreadScheduler-1,5,main]
3: Thread[RxCachedThreadScheduler-1,5,main]

It looks like the thread is not switched in the first case.