It depends on how elastic grocery prices are.

And so far it's 5.3% reduction in the spending of <16% of households on the drug for a total reduction of less than 1%. Compared to eg tariffs and general inflation, that's a bit hard to distinguish from noise.

There are other major factors also influencing grocery prices, such as tariffs. It may because that was are seeing a significant influence on price, but one that is counteracted by other influencers.

Your assumption of the existence of a grocery market competing on price might be wrong.