Sorry to hear that, but it is a problem with all dishwashers. One has to regularly remove the top cap and clean/soak the valve in a acidic solution like vinegar or de-limer to remove the calcium deposits in the tight area of the guide pin. Makes the valve bind. and not close. If pin or interior of the valve are scored or over worn you may have to replace it.
At idle the valve is open to air so that the rinse arms can drain and not have a back flow to the water system. When water pressure is supplied to the rinse the internal valve rises to close off outside venting. And that folks is why it’s called a vacuum breaker.