Enable SSML not working, SSML markup included in audio

- Extension name: text-to-speech
- Extension version: 0.1.4
- Configuration values (Shown in Firebase Extension configuration)

Collection path: "_textToSpeech"
Bucket name: "bucket.appspot.com"
Storage path: "_textToSpeech"
Enable ssml: yes
Language code: "en-GB"
Voice type: "Standard"
SSML Gender: "FEMALE"
Audio encoding: "MP3"
Enable per document overrides: no
voiceName: "en-GB-Standard-B"

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The Problem:

I enabled SSML in the configuration but when I input an SSML text value e.g.

To be
or not to be,
that is the question.

The audio file includes the markup e.g. "Less than speak greater than To be..."

If I refer to the Logs Explorer in GCP it shows the following...

ext-text-to-speech-textToSpeech Initializing text-to-speech extension with config:
jsonPayload: {
audioEncoding: "MP3"
bucketName: "bucket.appspot.com"
collectionPath: "_textToSpeech"
enablePerDocumentOverrides: false
languageCode: "en-GB"
location: "us-central1"
message: "Initializing text-to-speech extension with config:"
ssml: false
ssmlGender: "FEMALE"
storagePath: "_textToSpeech"
voiceName: "en-GB-Standard-B"
}

I noticed ssml property is set to false even though the dropdown in the configuration panel shows "yes" (lowercase).