Definitely not a bug. There is a piece of code in source that looks like this:

  function rotateLine(position) {
    if (position < halfabet) {
      context.rotate((-90 \* Math.PI) / 180);
    } else if (position > halfabet) {
      context.rotate((90 \* Math.PI) / 180);
    } else if (position === halfabet) {
      context.rotate((180 \* Math.PI) / 180);
    }
  }