The truth of the matter is that many supermarkets have had their way of working disrupted, nearly all of them work on a just in time basis. The disruption is for many reasons. Like hearing it or not, Brexit is one of them. Now some may insist that they have not experienced a shortage regarding their own personal shopping list, but one thing is for sure is that your personal cost of that shopping list is on the increase.
Supermarkets profit is based on the way they operate, once that operation is disrupted ( just in time) then gaps on the shelves is bad news, also goods arriving late will shorten the time those goods remain in store for you to buy, before they are dumped. More loss to the supermarket leading to higher prices to the customer.