It sounds crazy, but the cat might be fucking pissed about this abandonment and acting out. Cats hate change and this cat just went through at least two big ones: its home being invaded by the dog and then being ousted and thrust into a new completely foreign environment. I’d say try changing the litter, all the better if you can use the same litter the previous owner used to buy, try to get the same food it is used to as well, and give it a little time and some love and hopefully it’ll come around and stop.

Also yes I am judging the previous owner, that is just incorrigible.