Some fish has attacked the sailfin from the top to bite those holes. Tang aggressive action is generally a broadside battle and if a fin gets nipped it is likely to be the caudal fin. I’m not sure what a Foxface would do, I have not observed that they bite fins?
The clownfish are fin nippers and can be crazy aggressive when they want to. If you set up a camera to observe the tank when you leave the room my money is on one of the clownfish. I could be wrong, obviously.
I wonder what other people have observed who have these Tangs, Foxface and clowns in the same tank? I have them, but my terror is an old one-eyed damsel who swims in the middle and gives whatever bothers him.