Pooping outside the litter box

[kenna-99]

My two year old cat won’t stop pooping outside of the litter box. I got her as a kitten, and she wasn’t doing it initially, but started to within the first year and it has gradually gotten worse to the point where she uses the box rarely and usually poops on the floor. She never pees outside of the box though. She has chronic urinary tract issues, but eats specialized food and hasn't had a UTI in over a year.
I have another cat the same age and they get along well, and there are two litter boxes. The boxes are in the bathroom and she poops in lots of different places in the bathroom, but no other rooms. I’ve tried many different types of litters and different types and sizes of boxes to see if she was just picky, as well as having up to 6 litter boxes in the bathroom at one time. I wondered if it was a cleanliness issue, but even if the litter is 100% new and clean, she still poops on the floor.
I’ve tried positive and negative reinforcements. I’ve sprinkled catnip in the litter, given her treats when she does poop in the box, and punished her when she doesn’t, but it’s just gotten worse.
I’m assuming it’s a behavioral issue. My cats don’t get free range of the house when I’m gone, but the space they’re confined to is not small and is filled with comfy places and toys for them.
She’s generally a very stubborn cat. She knows the rules, but often breaks them, even right in front of me.
I feel like I’ve tried everything to fix this issue and I love her so much, but it’s exhausting to have to deal with this problem on a daily basis. Any advice would be greatly appreciated.

Sorry for the lengthy post, I just wanted to give as much info and context as possible.

[papa cat]

She’s generally a very stubborn cat. She knows the rules, but often breaks them, even right in front of me.

Cats don't abide by rules.There will be things you prefer them not to do but most cats will do things as they want to !

[Mollycat]

Cats also don't respond to punishment, I hope you're not doing that any more as it would most likely make behavioural issues worse not better. She is either distressed or protesting, and working with her to find out what it is will give you better results than any punishment.

Ok so from your point of view the area the cats are restricted to is generous, spacious, and has everything to meet their needs.
When they have free range when you're home, where are the areas they like best? Do they hang out in the places in the restricted zone or outside of it?
When they have free range and a stranger comes into the house, or they get spooked by a noise, do they run for places in the restricted zone or outside of it?
Where are they most relaxed, where do they sleep, in the zone or outside the zone?
Do they have other food bowls, scratch posts, litter trays, beds and toys outside the restriction zone? Do they use them?
When they have the choice, do they prefer to eat, sleep and play in the zone or outside of it?
Do the two cats have territory in the house? Silly question, all multi-cat homes have some level of territories! Who "owns" the area they are restricted to when you go out? Which of the two cats feels more comfortable in those places?

Several litter trays in the same room isn't all that helpful, do you have other litter trays around the house? Because if there is an issue with the bathroom, you will get the same result no matter how many trays are in there or what they are filled with.