If becomes widely known that the government can de-cloak Tor users, that will change the behaviour of their targets and so hurt their surveillance, so it's fairly common that the government wants to hide this fact. (same with stingrays, for example).

But just wanting to keep it secret is not enough. So they will claim that this has national security implications, saying some targets are terrorists. And courts defer very heavily to the government in this area, so the FBI might be successful.

Because just knowing that there is a break is enough to tip someone off, the FBI can and courts likely would classify a request for details as well as the details themselves.