Tell me about it. I didn't realize the "passive AI" bug existed until I read it on here, because it makes perfect sense to me for those units to not chase my horse archers. In my experience, when the unit starts running, it loses the shield bonus. At the very least, casualty rates go up when running. And, if they stop and backtrack to get back to their friends, they're turning their backs on the archers, which is even worse. I have had them try to corner me before, and learned quickly to be careful of that.