iOS Speech framework yields zero confidence for certain locales

When using the Speech framework, I am consistently noticing zero confidence values for certain locales (e.g. "vi-VN", "pt-PT", ...), while non-zero, accurate confidence values are returned for other locales (e.g. "ko-KR", "ja-JP", ...).

Looking at the documentation, the confidence would be zero if there was no recognition. However, when the zero confidence occurs, the formattedString of the bestTranscription is populated and accurate (same for each segment substring text).

I have tried instantiating the locales in various ways (language code only, language and region code, -/_ formatting, grabbing an instance directly off of the SFSpeechRecognizer.supportedLocales() array). I have also tried setting the defaultTaskHint of SFSpeechRecognizer and taskHint of SFSpeechRecognitionRequest to dictation.

I am stuck at this point. Any help would be appreciated. Thanks in advance :)

guard let locale = Locale(identifier: "vi-VN"),
      let recognizer = SFSpeechRecognizer(locale: locale),
      recognizer.isAvailable else {
  return
}

recognizer.defaultTaskHint = .dictation

let request = SFSpeechURLRecognitionRequest(url: ...)
request.contextualStrings = ...
request.shouldReportPartialResults = true
request.taskHint = .dictation
    
recognizer.recognitionTask(with: request) { (result, error) in
  ...
  if (result.isFinal) {
    let transcription = result.bestTranscription
    /// transcription.formattedString is correct
    /// all segments confidence values are 0, but with the properly recognized substring text.
  }
  ...
}