I have been wondering about that issue. I remember the cop shows of the 60/70s (e.g., Adam 12) where Miranda Rights were essential. Why was Flynn not protected by Miranda?
Miranda ONLY applicable at arrest...
They did not follow bureau procedure of telling Flynn that lying to the FBI is a crime, apparently. Something that would have been an issue in any trial on the charges, had one ever taken place.
Flynn was not in custody, or under arrest. They lulled him into an incrimination (non-incrimination actually) which they completely fabricated. They pretended that he was not a suspect, so Miranda did not apply.
That is why this was entrapment.