There, the heroes are scripted in such a way that they attack a specific cell on the map, and if the hero isn't there, they attack the next cell in their list. So, if you don't move away, you're left with one hero and a defeated army... He didn't even try to run to the well; he just immediately attacked... I even defeated him with the autobattle, and since the army isn't needed anymore in this mission, there's not much point in replaying it manually...