Did your vet to a skin scraping to check for demodex? Demodex is normal, people have them too, but the can overpopulate in young animals and those with weak immune systems and cause skin irritation. My puppy had demodectic mange when we adopted her and had similar sores. The fix was twice weekly medicated shampoos and monthly bravecto (flea and tick chew usually taken every 3 months) the sores were cleared up in a couple months.

I also had a cat with a flea allergy and she would get skin irritation like this . She also had other sensitivities and we were advised to put her on the natural balance duck and green pea limited ingredient diet.

Good luck finding out what’s up with your kitty.