Did he check her teeth. We had a cat that did this and found after a few years she had a bad tooth, removed it and she was a sweetheart again. Cats act out for various reasons, in pain you can’t see, change of litter, did you add anything (another pet to the mix). Perhaps the vet doesn’t think the arthritis is bad, but she feels differently. Can you move the box upstairs to see if it changes things.