An adipose fin can't regenerate, but a lot of times they aren't completely clipped or are missed entirely. I've caught fish with badly worn dorsal and/or caudal fins and intact adipose fins. These were obvious hatchery fish that were missed by the clipper. Not surprising considering the numbers of fish that are clipped each year. Your fish may have been one that got a botched clip job. Let's hope that the person who did it isn't a rabbi!