message_from_string return msg with wrong encode

Bug report

The text contain non ascii chars. The content of text/plain should show as above. While use email.message_from_string to parse the mime, message.get_payload(decode=True) decode "text/plain" part return wrong encode message.

Debug the code, found here https://github.com/python/cpython/blob/3.10/Lib/email/message.py#L278, get_payload return payload.encode('raw-unicode-escape'), but when I use message.get_charsets() it return utf-8, it doesn't match the encode charset. So the result is wrong. The final result is below, the charset is wrong, then I can't get the correct message.

Your environment
Python 3.9.10, macOS Catalina, version 10.15.5

Test code is below.
Runing command: python3 t.py TextBased.eml

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import logging
import sys
import email

def get_all_block(message, block_type = "text/plain"):
    content_type = message.get_content_type()
    main_type = message.get_content_maintype()
    if main_type == "multipart":
        if message.is_multipart():
            block = None
            for part in message.get_payload():
                result = get_all_block(part, block_type)
                if result:
                    if block is None:
                        block = result
                    else:
                        block += result
            return block
        else:
            return None
    elif content_type == block_type:
        result = message.get_payload(decode=True)
        if result is not None:
            charsets = message.get_charsets()
            print('charsets', charsets, result)
        return result
    else:
        return None

if __name__ == '__main__':
    fname = sys.argv[1]
    fp = open(fname, 'rb')
    mime = fp.read().decode('utf-8', errors='ignore')
    message = email.message_from_string(mime)
    text = get_all_block(message, "text/plain")

TextBased.txt

まず、レポートでリンクされている get_payload の実装周辺にある Lib/email/message.py から始め、提供されたスクリプトと添付ファイル TextBased.txt を使って動作を再現します。get_payload(decode=True) と、get_charsets() で取得したパートの宣言済み charset を比較します。text/plain のペイロードが宣言済みの charset と整合するエンコーディングで返され、関連する email テストにリグレッションテストのカバレッジが追加されれば完了です。