Flutter flutter_tts how do I lower the volume of audio in the background while playing tts?

Question

I'm using the plugin flutter_tts.

await flutterTts.setIosAudioCategory(IosTextToSpeechAudioCategory.playback,
    [
      //I've tried all of these options
      IosTextToSpeechAudioCategoryOptions.mixWithOthers,
      IosTextToSpeechAudioCategoryOptions.duckOthers,
      IosTextToSpeechAudioCategoryOptions.interruptSpokenAudioAndMixWithOthers,
      //the following are not relevant to my question I think
      IosTextToSpeechAudioCategoryOptions.allowBluetooth,
      IosTextToSpeechAudioCategoryOptions.allowBluetoothA2DP,
      IosTextToSpeechAudioCategoryOptions.defaultToSpeaker
    ],
    IosTextToSpeechAudioMode.defaultMode
);

The only result I get, is that audio from youtube for example (I have youtube premium, so I can play it in the background) pauses, and doesn't resume after my tts is done.
What I want, is that the volume of any background audio will only be reduced temporarily, while my tts plays, and later raised back again. I thought this one is supposed to do that, but it doesn't work, it completely pauses the background audio and doesn't resume it:

enum IosTextToSpeechAudioCategoryOptions {

  ...

  /// An option that reduces the volume of other audio session while audio
  /// from this session plays.
  duckOthers,

  ...
}