VoiceOver only works if TextFormField validator value changed

voiceover_on_submit.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();

  // Auto-enable accessibility for our Blind and Low Vision customers (see
  // https://docs.flutter.dev/development/accessibility-and-localization/accessibility#screen-readers).
  RendererBinding.instance!.setSemanticsEnabled(true);

  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    const appTitle = 'Form Validation Demo';

    return MaterialApp(
      title: appTitle,
      home: Scaffold(
        appBar: AppBar(
          title: const Text(appTitle),
        ),
        body: const MyCustomForm(),
      ),
    );
  }
}

class MyCustomForm extends StatefulWidget {
  const MyCustomForm({Key? key}) : super(key: key);

  @override
  State<MyCustomForm> createState() => _MyCustomFormState();
}

class _MyCustomFormState extends State<MyCustomForm> {
  final _formKey = GlobalKey<FormState>();

  bool _shouldAppendSpaceToErrorText = false;

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
          TextFormField(
            decoration: const InputDecoration(
              helperText: ' ', // Required to announce errorText (https://github.com/flutter/flutter/issues/99715).
              labelText: 'Username',
            ),
            validator: (String? value) {
              if (value == null || value.isEmpty) {
                return 'Please enter your username' + (_shouldAppendSpaceToErrorText ? ' ' : '');
              }
              return null;
            },
          ),
          TextButton(
            onPressed: () {
              // Ensure that the errorText changes so that it is announced.
              _shouldAppendSpaceToErrorText = !_shouldAppendSpaceToErrorText;

              if (_formKey.currentState!.validate()) {
                ScaffoldMessenger.of(context).showSnackBar(
                  const SnackBar(content: Text('Submitting...')),
                );
              }
            },
            child: const Text('Submit'),
          ),
        ],
      ),
    );
  }
}

• Issue: flutter/flutter#99715
• Flutter version: 2.10.3
• Tested device: Mac OS Big Sur 11.6 M1
  Chrome 98.0.4758.109 (Official Build) (arm64)
• Result: Work normally with second author's temporary solution
• Replicable:
  • Yes (for first code sample)
  • No (for second code sample)

(Demo the second code sample)
https://user-images.githubusercontent.com/29337364/157238232-90af4153-eb1f-46c0-b487-16a3bf00eac4.mp4