IF the mandrel is bent, you will, after a few passes, be turning an out of round, but consistent, shape. The chatter stops.
Chatter is vibration that is not "regular".
If your finished pens are not round, but oval as viewed looking into a tube, THEN you have a bent mandrel. Suggest using the "pro" mandrel and doing one half a pen at a time. I've not had an oval pen since I switched to this method.