My sentiments exactly regarding the Tesco changeovers, there is probably some ludicrous railway specific agreement buried behind it as per.
Tesco isn't a moving 2/3m2 serving area where all of its stock is loaded and unloaded in to carts, nor does it have one point of sale / till / device to process transactions, nor does staff change shifts in the short space of a station call.
It's more than reasonable that on a journey where there's a crew change that the shop will close just before the changeover stop and reopen shortly after while the crew log out / cash up the epos (even if card only, needs to be done between crews), possibly count stock (appreciate not always the case), and make way for a top off order which is to be loaded also in the short space of time that the train is in the station. The staff then may also need to collect their belongings from an alternative location on the train, there may be mess that should be tidied so as to not hand over a dirty workplace to the next crew, and any number of smaller more insignificant tasks that need to be done before the new crew start.
So no, it's not a ludicrous railway agreement it's part and parcel of running a service on a moving train.