After recently messing around with a couple custom (hell, and even one from the core game files) map for tactical battles, I noticed a problem. It's like if the map isn't an open battlefield, the AI gets confused. For instance, there was a choke point on this one map I was playing. At one point, there was only one square to advance forward. The AI, before even getting to this spot, then goes off in another direction, to finally turn around and actually make its way towards me. It's really strange. There was another one where the AI actually went away from me (not as in running away, it was a strong monster, and the battle just started), again, only to turn around and finally make its way to me. It seems it has a real hard time if there are multiple enemies, and less passable tiles in front of it.
EDIT: I took a screenshot of each turn (I just kept pressing enter to see how they moved). Don't mind the black splotches, it's a map in progress
Turn 1 (Start of battle)
Turn 2: (So far, so good. They've moved up without a hitch so far)
Turn 3: (Ok, some funkiness begins. The skeleton in the back didn't move, even though there's room to move up.)
Turn 4: Ok, so now you see the other zombie wandering off to the woods, and one inching towards me. The skeleton finally moved here as well.
Turn 5: Ok, so the zombie turns around, and heads towards me again, and the skeleton is still on his way over too.
Turn 6: Apparemtly, that zombie just likes to go back and forth over there. It's like if it hits a chokepoint, it doesn't realize that it can't move because of another unit (instead of an unpassable tile which never changes), so it tries to go another way.