TYPO3 Core

If you add HTML to the email, initializeStandaloneView() is called twice, first for plaintext, second for html, but the templatePathAndFilename option is overwritten in the method, and then {@format} is no longer available, so the HTML part also us the plaintext template.

$this->options['templatePathAndFilename'] = strtr($this->options['templatePathAndFilename'], [
   '{@format}' => $format
]);