My biggest suspicion would be that this guy is tail-biting. Usually seen in veiltails since they grow the longest fins...but any long-finned bettas are susceptible. Not really clear what causes this, there seems to be all sorts of theories but none of which is concrete, IMO.
BTW, that is probably one of the best looking double-tail bettas I have ever seen. And I'm not even a big fan of double-tail bettas.
It just seems like I would have seen him nipping at his tail at least once, but I will keep an eye on him. I turned the filter all the way down and might try removing the spiralis if he doesn't start to improve. One of the shreds is very far down, I don't see how he could have done that to himself. Seems like if he was biting his tail, it would be little bits off the edge, but I'm not entirely certain. He is nice looking and has such a silly personality. He is busy shrimp hunting right now, so that should keep his mind off tail biting if it is the problem. His "strike" is pretty ridiculously slow with that big silly tail. The shrimp get away 99% of the time, but he is persistent.