Why Observable.fromIterable miss some data in some case? - rx-java2

Question

I write a sample to test flatMap operator in RxJava2, below is the code snippet:
Observable<Integer> observable = ObservableCreate.create(new ObservableOnSubscribe<Integer>() {
#Override
public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
emitter.onNext(1);
emitter.onNext(2);
emitter.onNext(3);
emitter.onNext(4);
emitter.onComplete();
}
});
Disposable disposable = observable.flatMap(new Function<Integer, ObservableSource<String>>() {
#Override
public ObservableSource<String> apply(Integer i) throws Exception {
List<String> list = new ArrayList<>(3);
for(int j = 0; j < 3; j++) {
list.add("I am from apply:" + i);
//list.add("I am from apply:" + i + "_" + j);
}
log("apply: " + listToString(list));
return Observable.fromIterable(list);
}
})
.flatMap(new Function<String, ObservableSource<String>>() {
#Override
public ObservableSource<String> apply(String s) throws Exception {
return Observable.create(new ObservableOnSubscribe<String>() {
#Override
public void subscribe(ObservableEmitter<String> emitter) throws Exception {
emitter.onNext("flatMap--" + s);
emitter.onComplete();
}
});
}
})
.subscribe(
s -> log("accept: " + s)
, throwable -> log(throwable.getMessage())
, () -> log("complete!")
);
Do you think what logs will print? Is it going to something like the following?
apply: [I am from apply:1, I am from apply:1, I am from apply:1]
accept: flatMap--I am from apply:1
accept: flatMap--I am from apply:1
accept: flatMap--I am from apply:1
apply: [I am from apply:2, I am from apply:2, I am from apply:2]
accept: flatMap--I am from apply:2
accept: flatMap--I am from apply:2
accept: flatMap--I am from apply:2
apply: [I am from apply:3, I am from apply:3, I am from apply:3]
accept: flatMap--I am from apply:3
accept: flatMap--I am from apply:3
accept: flatMap--I am from apply:3
apply: [I am from apply:4, I am from apply:4, I am from apply:4]
accept: flatMap--I am from apply:4
accept: flatMap--I am from apply:4
accept: flatMap--I am from apply:4
complete!
I also think that I'll get these logs, but I am wrong, I only get two for each string, just like this:
apply: [I am from apply:1, I am from apply:1, I am from apply:1]
accept: flatMap--I am from apply:1
accept: flatMap--I am from apply:1
apply: [I am from apply:2, I am from apply:2, I am from apply:2]
accept: flatMap--I am from apply:2
accept: flatMap--I am from apply:2
apply: [I am from apply:3, I am from apply:3, I am from apply:3]
accept: flatMap--I am from apply:3
accept: flatMap--I am from apply:3
apply: [I am from apply:4, I am from apply:4, I am from apply:4]
accept: flatMap--I am from apply:4
accept: flatMap--I am from apply:4
complete!
I try to find the reason of this, so I add some breakpoints and debug step in step, unfortunately, I get the right result after debugging. I am so confused, does fromIterable operator miss some data in some case?