Midi note play/stop note not working if inside of Future.Delayed

[IsaacWallis]

I can't seem to schedule notes on/off using Future.delayed. Am I doing something wrong?

Future load(String asset, FlutterMidi midiPlayer) async {
  midiPlayer.unmute(); // Optionally Unmute
  ByteData _byte = await rootBundle.load(asset);
  await midiPlayer.prepare(sf2: _byte);
}

void playNotes(String asset) async {
  FlutterMidi midiPlayer = FlutterMidi();
  await load(asset, midiPlayer).then((value) {
    midiPlayer.playMidiNote(midi: 60);
    Future.delayed(Duration(seconds: 1), () {
      print ('stops now'); // this prints, but the note continues playing as if there were no noteOff 
      midiPlayer.stopMidiNote(midi: 60);
    });
     
    // if we comment out the previous Future.delayed and uncomment the next two lines, 
    // the noteOff comes at the correct time (but of course sleep blocks the main isolate,
    // breaking the UI; so we can't use this method) 

    // sleep(Duration(seconds: 1));
    // midiPlayer.stopMidiNote(midi: 60);
  });
}

I have tried spawning a new isolate so I could use sleep, but there were many error logs and then neither notes on nor notes off would work at all.

[IsaacWallis]

This appears to be caused by the fact I am using this inside the then of load? Perhaps, something is still not initialized correctly by the time that future resolves? If I move the note on and note off into a separate method which is called upon a button click, both the note on and the note off work correctly.