It's definitely a bug, as the ai are the attackers in this case, so they should attack. If they're not programmed to attack when the ai is the attacking side, they would always lose on account of the timer running out. This obviously creates problems when playing without the timer.
I had the same thing happen to me, even charging my cav into them wouldn't lure the enemy across the bridge, luckily my army still could retreat without the army vanishing.