Noki has been eating Scooters left over wet food and we think he's allergic to something in it. He's licking the hair off his butt so he has to wear the donut. We know he's allergic to wheat but neither his dry food or Scooters wet food has any so we're guessing he's also allergic to the soy in the wet food. Trying to keep him from eating the leftovers and also switching it up with a different brand.
Try barfmenu...raw meat with some supplements like Taurine. We don't give our cat processed dry food or anything with filler like wheat or vegetables. It is a bit more expensive though. My cat only likes the chicken version. He almost 18 now, blood values good like a younger cat.
Ask your vet before you take any advice from the internet though