Pig shopkeeper suddenly disappears

As the title. When I bought something in shop in multiple times without going out, she suddenly disappears into thin air.

Edit : As far as I remember, she left a manure in the corner of the shop, which is not a behavior for shopkeeping pigs, so I was wondering about this is related to disappearance of that pig. I picked up that manure, and buying something, then she comes to refill the product, and disappears!