I've been finding that on tactical maps with obstructed terrain (mountains, forests, chasms, etc) that often, melee units will get stuck behind the obstacles, apparently in an effort to reach their target by traveling the shortest route rather than move around to reach their target. I haven't seen this in previous versions.
In the image posted below, the skeleton group (and other mobs, already killed) has been stuck behind the chasm for about 20 tactical turns now.