This is my edited version of Bannered Mare Immersive Sexual Playground V 8.01 a great LE mod by t.ara, ported to SE with some minor edits for immersion.   The Bannered Mare Immersive Sexual Playground is a great mod that has been in my load order for a few years.  One of the things I noticed was that nobody except the player ever goes down the stairs.  NPCs go upstairs, but no one ever goes down.  Upon checking, I realized that it's because the navmesh stops at the top of the stairs.