Funny state behavior that I can't reproduce

funny_state_behavior.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<int> productQtys = [10];
  String status = 'Status:\n';
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Demo"),
      ),
      body: Center(
        child: Text(
          status,
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            int index = 0;
//             print(productQtys);
            status += '\n$productQtys';

            if (productQtys[index] != 1) {
              productQtys[index] = productQtys[index] - 1;
            }

//             print(index);
            status += '\n$index';

//             print(productQtys);
            status += '\n$productQtys';
          });
        },
        tooltip: 'Decrement',
        child: Icon(Icons.remove),
      ),
    );
  }
}