From address gets ignored in LegacyMailerHelper

I can't see any code in Drupal Core or Swiftmailer that looks in $message['from']. Swiftmailer has a very clear comment:

      // \Drupal\Core\Mail\Plugin\Mail\PhpMail respects $message['to'] but for
      // 'from' and 'reply-to' it uses the headers (which are set in
      // MailManager::doMail). Replicate that behavior here.

So perhaps the code in Commerce is non-standard??