I am not an expert but with many people here at PT I learned that introducing a cat to another is a longer process and can have its setbacks.

However I have some experience in cats that are very picky with their toilet.

Do they have 2 litterboxes? It's the minimum. And as Devon said: always clean it after they used it (The good thing: mine complein first to the service before they look for other places)

In addition - watch what they are doing. Our litterboxes were covered and Tigris used to attack Filou in the box. Because of the cover Filou couldn't escape and started to avoid the litterbox. Now I have open boxes which needs more cleaning, but the pee is in and not in the basket with the dirty laundry

Good luck in finding the reason and in introducing Kizia to Lucy