AI is either not updating its pathing or it intentionally ignores fallout, not sure which. I detonated a regular nuke in empty hexes outside my border for my Roman Legions to play with, and roughly ten turns later Roosevelt waltzed an organized mass of 6+ Apostles and missionaries through the radioactive fallout zone despite perfectly open pathing on all sides to go around. 
Almost all of them melted in the attempt.
v 1.0.0.38
Almost all of them melted in the attempt.v 1.0.0.38