I have a rescue that the same exact thing happened to. He had fin rot when I got him, but after I treated him and he showed some regrowth he fins suddenly looked ragged again. This happened several times with me treating him as if it were fin rot, and I finally caught him in the act of biting them one day. By now I have tried EVERYTHING to no avail and I've pretty much given up hope that the issue will ever be resolved. Besides this he is happy, healthy, eats wel,l and I have had him for a year in December.
My advice is that you watch it to make sure that it doesn't get worse, as well as observe his behavior to look for signs of fin biting. If it seems to deteriorate steadily than suspect fin rot, but if it begins to heal or the fins show missing chunks only sporadically, then suspect biting. Good luck!