I just lost one of my cats to losing weight she was a 8 year old spayed bengal cat. She lost 75% of her weight in 4 months. The vet ran every test possible and could find nothing wrong wit her. After coming back from the vet she died a week later.
I had something similar. My F4 Savannah Jayboy got sick back in Sept and lost 50% of his body weight in less than 8 weeks. The vet did all the blood work and x-rays and could find nothing wrong either. He got weaker and weaker and passed away in my lap 2 day before Christmas. He was just shy of 9 yrs old. I beat myself up over it for several months wondering what more I could have done but what can you do when the Vet can't tell you whats wrong. My sympathies go out to you also for your loss.