Incorrect item delivered with our ASIN

We have a product on sale for almost a year, with good reviews and reasonable sales. However, in December we received 8 complaints saying the incorrect item was delivered but it had our ASIN and product description on the label.

The items we sell are adult colouring and activity books, but the buyers described the item delivered as either a Toilet/Lunch/freezer type bag with ducks on it.

We do not sell any item even close to that description, but all customers said it had our ASIN and description on the label. We have opened a case with Amazon but so far we've had no joy. It isn't affecting our account health yet but we don't like the thought that people bought these as a Christmas stocking filler and got the wrong product.