Just a small thing that happened to me now (yup, i'm using your latest AI this time)
I was fighting a very though bridge defence battle as Mordor against Gondor.
I deployed my troops as in the image below, and charged the units that engaged my infantry, routing them. They had those spearmen (in pic) in the bridge, who stopped in the middle of it. The units coming behind them were destroyed, but they just stood there. When there were only the spearmen left one unit left the bridge and got routed, followed by a second unit of spearmen. But the last 3 just stood there no matter what. I tried luring them out of the bridge but nothing worked, guess your AI has reached a point where they start learning from their mistakes . They're almost immune to my archers, so i just waited for the end of battle.