Navmesh generating under objects

So I have a plane and placed a box on top of a plane ( intersecting ), the problem im running into is the navmesh is generating both on top and underneath the box. so if my navmesh agent is going up a ramp it cant decide which navmesh to use. Is there a way to keep the navmesh on top of the box only? I thought usually navmesh would make the walkable area disabled under solid objects

Did you move the box recently without rebaking it? or if this isnt just a box is the collider convex?