I don't know if this is intended behavior, but I've been noticing that soldiers will not react to shots that hit them from behind. This is contrary to aliens, who will react to shots that hit them from behind. I am fairly certain that this behavior was in the original game, though it could have been difficult to tell given how uncommon it was for a soldier to survive a hit, let alone react to it. But I have soldiers with high reactions fighting aliens with low reactions, and my soldiers will definitely shoot back when they can see the alien, but they will not shoot back at an alien they are facing away from.
Shouldn't X-Com soldiers be able to react to being shot from behind?