My cat likes to scratch on this weird vinyl/fake leather chair that I have. I've tried covering it in tape, and he still scratches it. One day, I realized that whenever he scratches it, he almost immediately turns to look at me, so I started to wonder if he might be doing it for attention. On top of that, as soon as I tell him "no", he usually walks over to me, looking for attention. It took a lot of patience, but since the chair was already ruined, I decided to try just totally ignoring him whenever he scratched the chair, and giving him positive attention when he scratched on his post. Somehow, it actually worked!

Another suggestion would be to put a scratching post as close as possible to the couch corners as possible, and when you do catch her scratching the couch, very calmly reposition her to the post. Praise her for using the post, completely ignore her for scratching the couch.

I know you mentioned that she has a cat tree nearby, but the geometry of the couch might be giving her a better stretch, she may like the material better, and/or she may be looking for a way to better claim the couch as part of her territory, as a scratching is also an important way that they leave their sent on things.

https://www.jacksongalaxy.com/blogs/news/how-to-stop-your-cats-from-scratching-furniture