Firemonkey debugger problem for string value

I cannot see the result of a string assignment in the debugger:

procedure TfMMain.AudioPlay(WordStr: string);
var
  AudioFile: string;
begin
  AudioFile := fMMain.cdsWords.FieldByName('AudioName').AsString;
  if AudioFile = '' then
  begin
    MessageDlg('Audio not available for this word',TMsgDlgType.mtInformation,
      [TMsgDlgBtn.mbOK],0);
    exit;
  end;
end;

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The debugger tells me the correct values for fMMain.cdsWords.FieldByName('AudioName').AsString;
but displays garbage value for the local value "AudioFile".

Is this a known problem / feature, or am I doing something wrong?

The debugger works OK if I attach the value directly to the form.