Kitty keeps pooping on the stairs in protest to her diet

Cat (female, about 7YO) keeps pooping on my stairs 2-3 days a week. I think she is protesting her diet and I don't know how to stop it.

First, I feel confident there is no medical issue. She saw the vet within the last 60 days and everything was normal. Pooping in protest is also a somewhat normal behavior for her, and she has done it in the past when there's a disruption in the house (people visiting, construction noises outside, breakfast was late) just not this consistently before.

We did have to put kitty on a stricter diet recently. She was technically already on a diet and has been for about a year, but we just restricted the amount a little further because she wasn't losing any weight. The change was recommended by our vet. She's not oh lawd chonky, but definitely firmly in the overweight category. The diet does seem to be going well now after the reduction in amount, and I actually noticed her chasing and rough-housing a toy without human encouragement for the first time in a long time. She is able to clean herself better too, and her fur has been softer. Yay!

The kitty litter is not dirty or too low. I have found poop on the stairs, then checked the box to find it full of litter and nothing else. We went through a whole exercise to find the specific kitty litter she likes when we first got her in 2020, and are still using it, so I don't think it's the brand of litter either. She has never once peed outside the box, only pooped. She also does poop in the box most of the time.

I am cleaning the areas where she poops with a pet focused carpet cleaner. She doesn't poop in the same spot each time, but always does it somewhere on the stairs.

This is the 6th cat I've owned/lived with and I try not to get upset about accidents, but her behavior is becoming very consistent and frustrating, and she's just been to the vet, so I don't think there's a medical cause. She might be on this diet for a while and I don't want to step on her little landmines on the stairs every morning.

Any suggestions to keep her pooping in her box? Thanks in advance!