From the discription the beater is aluminum. The aluminum will react with dishwasher detergent if the detergent is not aluminum safe. I worked in commercial foodservice for many years and I had to make sure the dishwasher detergent was aluminum safe so everything could be cleaned in the dishwashers.
I am my own dishwasher at home so I am not sure if an aluminum safe dishwasher detergent is on the retail market.
KitchenAid is a good brand so I would call them to see what can be done.