I must try Waitrose then. All of the other stores seem to have now got systems which automatically remove any special offers on discounted items :-(
Before most stores worked out how to stop their computers from making 'silly' reductions, I once bought some short-dated yoghurts in Asda where (because of an anomaly of the type you've experienced) my total bill was less than if I'd not bought the yoghurts at all!