I don’t think the Flynn case has anything to do with FISA warrants. He may have been subject to surveillance under a FISA warrant earlier in 2016, but his conversations with the Russian ambassador that were the basis of the FBI’s interrogation of him were not covered under a FISA warrant. As a foreign diplomat the Russian ambassador would have been under direct surveillance by U.S. intelligence agencies as a matter of course, and since he’s not a U.S. citizen he doesn’t have any legal protection under U.S. law that would have required anyone to obtain a FISA warrant in the first place.
The rampant unmasking may have impact, though. Especially given his actual role at the time (cabinet designee)
The FBI can interview you without your attorney present, fake their records, and then send you to prison on fake evidence. That’s what happened to Flynn.