That’s nothing to do with pathfinding, and nothing to do with OpenMW. The idle animation that NPC uses ends in a different place than it starts, so after a few loops, they fall off the platform. That happens both with the original engine and with OpenMW as both engines faithfully do what the game data tells them to do. You need a bugfix mod like Patch for Purists to replace the animation with a seamlessly-looping one, and you should be using a patch mod like that for a lot of other reasons, too, as the game data contains plenty of bugs.