My guess would be that the cat might be trying to mark or reclaim her territory. I had to let my cats outside bc when i brought my babies home they would pee on everything that belong to them. I went through this with 2 separate kids and cats both cats neutered males and had never used the potty anywhere other than the litter box. So even though they have adjusted to one another the cat may just be trying to show dominance or possibly jealousy.
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