I've had this happen on both islands and in all types of terrain, although it is more common in urban areas. Sometimes there are bodies in front of them but I've got tons of screenshots with AI squaddies who are refusing to cross clear ground as well.
Arma 2 DID have its own AI pathing issues, especially with vehicles and inside buildings- I've managed that for years without this kind of trouble (so I'm not just a run-&-gunner new to the series or something) but this is much much worse. I say this with the greatest amount of respect and appreciation for what you do, Arma 3 really shows your hard work, but the AI pathing is just killer.