It looks like the AI is trying to walk down, but can’t because it’s constrained to a plane.
As mentioned before, you’ll want a narrow navigation mesh so that the AI won’t try to wander off. If you tick navigation in the show menu of the map editor, it will show the mesh AI uses to move around. for a 2.5D game, it should look something like this:
If your whole level is green, that’s probably the problem. Or if there’s no navmesh, the NPC is probably just stuck in the default 90° rotation, and you’ll need to add a navmesh bounds volume.