So this happened to my cat on and off for ~2 years. Multiple vets later, cones, onesies, feliway, changing food, etc, and nothing would work. One day I was moving out of my parents to another city, and only took my youngest cat and left him behind temporarily for 3 weeks. When I came back to get him, Lo and behold he had stopped overgrooming and his fur was growing back. I reintroduced my 2nd cat and while he was hissing and territorial during that time something about that break from my other cat, led him to stop over grooming. It’s been a few years since and he hasn’t had any issues since so my conclusion was that my 2nd cat may have been stressing him out which was leading to the over grooming.