I’m trying to use the write method on AVSpeechSynthesizer to capture both audio data and the markers data from synthesized speech. However I never see the markers callback invoked.

Minimal example code:

var synthesizer = AVSpeechSynthesizer()
synthesizer.mixToTelephonyUplink = false
synthesizer.usesApplicationAudioSession = false

synthesizer.write(AVSpeechUtterance(string: "Hello world"), toBufferCallback: { buffer in
    if let buffer = buffer as? AVAudioPCMBuffer {
        print("write (buffer.frameCapacity > 1)")
    } else {
        print("could not create buffer or buffer empty")
    }
}, toMarkerCallback: { makers in
    print("markers")
})

Tested on device and simulator. Buffers are provided until the end of the audio data is reached but toMarkerCallback is never called

Is there something else I need to do to get the markers as well?