Why captured Allied IFF knowledge had limited effect

Did the United States realize during the war that Japan could spoof Allied identification friend-or-foe signals?

Not fully, but Japan used the capability too little to make it decisive, and late-war pilot losses limited its practical exploitation.

The United States did not fully solve the issue, but Japan rarely exploited the capability at decisive scale. By the time IFF deception could have mattered greatly around Okinawa, too few experienced Japanese pilots remained to apply it consistently. The answer again ties technical knowledge to the trained personnel needed to turn it into operational effect.