Just kidding. Well, almost...
I know there was quite a discussion over this topic during FE beta. I think this is NOT the same problem (actually, I saw a Butchermen army dispatching some AI defenders later that game).
The problem: AI and monster stack are occupying the same tile without apparent fight or casualties.
The (possible) cause: moves taking place on the same time period with one of the armies targetting a tile further away and thus not checking the intermediary ending tile...

On this screenshot, you can (not) see a Skirmisher unit just below Sssrip's stack. The following turn, each army went its own way, without any will to fight.