I have had similar problems. I have a male betta and the water parameters are completely fine. I don’t know why in the world his fins sometimes get messed up, to be honest I have had him for almost a year now, and he has never had perfect fins. When I treat him with Melafix, his fins just get worse. So I stopped the Melafix and just let him be the way he is. He eats fine; he is perfectly healthy as far as I can tell. I've never found out what it is that’s causing this; I thought I was the only person who had this problem.