AWS Transcribe: missing speaker_label when using custom vocabulary

I am testing AWS Transcribe for my PoC project. When using "speaker partitioning" and "custom vocabulary" at the same time, the results.items section does not contain speaker_label, only for the item that matches the custom vocabulary:

{
  "jobName": "transcription-job",
  "accountId": "MY_ACCOUNT_ID",
  "status": "COMPLETED",
  "results": {
    "transcripts": [ ... ],
    "speaker_labels": [ ... ],
    "items": [
      {
        "type": "pronunciation",
        "alternatives": [
          { "confidence": "0.8", "content": "WORD_INCLUDED_IN_VOCABULARY" }
        ],
        "start_time": "1.23",
        "end_time": "1.419" // speaker_label is not present only for vocabulary matches
      },
      {
        "type": "pronunciation",
        "alternatives": [
          { "confidence": "0.8", "content": "WORD_NOT_INCLUDED_IN_VOCABULARY" }
        ],
        "start_time": "1.540",
        "end_time": "2.210",
        "speaker_label": "spk_0" // speaker_label is present, as expected
      }
    ]
  }
}

Is this an expected behavior, or am I missing something? I am using @aws-sdk/client-transcribe@3.523.0 and ja-JP for the language settings. The request is sent like this:

await transcribeClient.send(
  new StartTranscriptionJobCommand({
    TranscriptionJobName: jobName,
    Media: { MediaFileUri: S3_URI },
    OutputBucketName: bucketName,
    OutputKey: "my-output-files/",
    LanguageCode: "ja-JP",
    Settings: {
      ShowSpeakerLabels: true,
      MaxSpeakerLabels: 10,
      VocabularyName: awsVocabularyName,
    },
  })
);