Observable.concat() have unexpected result

this is my code

import 'package:rxdart/rxdart.dart';

void main() async {
  print("main start");
  Observable.periodic(Duration(seconds: 1), (o) => print("${o + 1} second")).listen(null);
  var result = await testConcat().toList();
  print(result);
}

Observable testConcat() {
  return Observable.concat([
    Observable.timer(50, Duration(seconds: 5))
        .doOnListen(() => print("observable1 listened"))
        .doOnData((o) => print("observable1 get data")),
    Observable.timer(100, Duration(seconds: 3))
        .doOnListen(() => print("observable2 listened"))
        .doOnData((o) => print("observable2 get data")),
  ]);
}

and it is result

main start
observable1 listened
1 second
2 second
3 second
4 second
5 second
observable1 get data
observable2 listened
observable2 get data
[50, 100]

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