It isn't something normally associated with paralysis, so it sounds like more of just a doggy thing?
I tried looking up dog treat recalls for 2017. If there was a recall, any products should have been pulled from the shelf but maybe something was missed? You can look at this list. I didn't read the whole list but the first one mentions tummy trouble.https://www.petful.com/recall-lists/dog-food-recalls/
Another idea might be, did your dog drink from a community water bowl at the store? I know at my PetsMart there is a water bowl underneath the drinking fountain back by the restrooms, for example. If he did, maybe he picked up some kind of intestinal parasite? I wonder if the fecal test the vet ran was comprehensive or if maybe it just checked for certain common things. You could ask if there are any other fecal tests that might test for different parasites or whatever?
I hope you'll post anything you find out.
I wonder if probiotics would help...?