Speech Recognition: Alternate substrings always empty, confidence 0 most of the time

I'm trying to work with speech recognition in iOS, but I guess we're not getting friends...

I've set up an SFSpeechRegonizer and set its delegate accordingly. Now I implement the delegate method as follows:

func speechRecognitionTask(_ task: SFSpeechRecognitionTask, didHypothesizeTranscription transcription: SFTranscription) {
    print("transcription:         \(transcription.formattedString)")
    print("alternativeSubstrings: \(transcription.segments.map { $0.alternativeSubstrings })")
    print("confidence:            \(transcription.segments.map { $0.confidence })")
}

This prints out something as follows whenever the delegate method is called:

transcription:         Space – the final frontier. These are the voyages…
alternativeSubstrings: [[], [], [], [], [], [], [], [], []]
confidence:            [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

The transcription is good. However, the alternativeSubstrings arrays are always empty and the confidence values are 0 most of the time. Every now and then they are non-zero – but it's unpredictable when this happens:

confidence:            [0.56, 0.049, 0.558, 0.545, 0.476, 0.6, 0.654, 0.647, 0.829]

Why are there no alternativeSubstrings, why is the confidence 0 most of the time and how can I fix this? I Apple's Speech library simply broken or what's wrong?