Cat pees Under the door, please help (long story)

Hi! Im 26, posting from Estonia, parent of my two female cats (5, 6 years), both spayed. We already visited and contacted two different vet clinics on this topic.

Almost a year ago in our previous apartment the older cat started peeing under the apartment door. As soon as we saw that the first time, we checked the litter boxes, checked her, nothing was wrong, so we called the vet and they told us to keep an eye on her and see if anything has changed.

So we did that, she was perfectly fine, was behaving absolutely usual, ate, drank water and went to do her business in the litter box.

In a few days she peed under the door again. She did so while we were at work. We started freaking out, cause she never did that in her life so we took her to the vet. They did an ultrasound and some blood work. Everything came back great. We were instructed to separate two litter boxes on different rooms and add one more with a different type of litter. So we did that and she started peeing in the spot where the second litter box was. And never in the new sand.

So we put everything back, she stopped doing that for some time. Then in a few months again, under the door. And ever since then she does that a few times a week. Even in new apartment (the layout is the same as the last tho)

Nobody seems to be able to help us. Otherwise she behaves just fine, she eats, she drinks usual amounts of water. She likes the sand (we have tried different options). Her food works for her. She’s friends with the second cat, they play together. There were no changes when she started doing that.

She can do that at night, or while we at work or even while we are at home.

Younger cat has never done anything like that.

Please help, we can’t figure out what to do and how to help her and ourselves but that affects our lives more and more with every time she does that and we are confused and worried 😭