I would think this kind of repair required tearing up the concrete floor. With six stores closing at one time it sounds to me like they hired the same plumbing contractor that didn’t use the right pipes. OR, being a fan of importing Chinese junk they used pipes from China that are contaminated with some terrible chemical they are keeping under wraps to avoid lawsuits. They’ll have to clear all the shelving, cut the floor up, make the plumbing repairs, replace the floor, then restock all the shelves. I could see that taking 6 months.
They've moved entire sections of store around in mere days to accommodate new floor plans. The plumbing is likely peripheral, at the front and back of the stores, and maybe on the sides. I doubt there would be more than a few pipes under the main floor, all their sprinklers for veggies are in units coming down from the ceiling. There's no way a drainage issue would take six months to deal with. It certainly wasn't a supply issue, or the employees and customers would have noticed that immediately.
When they opened the newest Walmart to me, they stocked the non-perishables about a week before, with the perishables coming in within two days of opening. No way this is plumbing. It may certainly be any number of things, but it's not plumbing.