🇯🇵Japan @iwahashi

I created following patch to fix this issue.

--- src/Utility/MimeMailFormatHelper.php.orig 2023-03-24 06:16:53.000000000 +0900
+++ src/Utility/MimeMailFormatHelper.php    2023-06-09 15:00:09.844483234 +0900
@@ -792,6 +792,7 @@
     // Run all headers through mime_header_encode() to convert non-ASCII
     // characters to an RFC compliant string, similar to drupal_mail().
     foreach ($headers as $field_name => $field_body) {
+      $field_body = str_replace("\r\n", '', $field_body);
       $headers[$field_name] = (new UnstructuredHeader($field_name, $field_body))->getBodyAsString();
     }

I'll explain why this patch is valid.
Before the foreach loop, $headers['From'] in my case is

=?utf-8?Q?Drupal9=E9=96=8B=E7=99=BA=E7=94=A8=E3=82=B5=E3=82=A4?=
 =?utf-8?Q?=E3=83=88?= <root@drupal9-devel.XXX>

And after the foreach loop, it becomes without the patch

=?utf-8?Q?=3D=3Futf-8=3FQ=3FDrupal9=3DE9=3D96=3D8B=3DE7=3D99?=
 =?utf-8?Q?=3DBA=3DE7=3D94=3DA8=3DE3=3D82=3DB5=3DE3=3D82=3DA4=3F=3D?=
 =?utf-8?Q??= =?utf-8?Q?=E3=83=88?= <root@drupal9-devel.XXX>

The getBodyAsString() function flow is as follows.
vendor/symfony/mime/Header/UnstructuredHeader.php: getBodyAsString() → $this->encodeWords()
vendor/symfony/mime/Header/AbstractHeader.php: encodeWords() → $this->tokenNeedsEncoding()
vendor/symfony/mime/Header/AbstractHeader.php: tokenNeedsEncoding() → preg_match()

protected function tokenNeedsEncoding(string $token): bool
    {
        return (bool) preg_match('~[\x00-\x08\x10-\x19\x7F-\xFF\r\n]~', $token);
    }

If $field_body contains LF or CR, UnstructuredHeader()->getBodyAsString() encodes $field_body again.
So, we must remove extra LF and CR before UnstructuredHeader()->getBodyAsString().