Using TextField on Samsung devices, the behavior when converting from Japanese to 2-letter symbols is bad.
The same phenomenon did not occur when I used Google Gboard, so I think this is a problem specific to Samsung's software keyboard .

Steps to Reproduce

1. Execute flutter run on the code sample
2. Type "かっこ" in Japanese
3. Tap the line break symbol in the lower right corner of software keyboard.

Expected results:
Text input is confirmed. Like this word app.

Actual results:
Text input is not confirmed.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyHomePage());
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold(
        body: Center(
          child: TextField(maxLines: null),
        ),
      ),
    );
  }
}