How do I prevent a recast graph from being generated on top of my ceiling?

I’m trying to generate a recast graph based on colliders inside an enclosed room. If I include the ceiling collider in the mask then it correctly blocks areas inside the room that are two low to walk through, but it also generates a separate region on top of the ceiling collider outside the graph bounds:

Why is the generator creating triangles outside the specified bounds?

If I exclude the ceiling from the bounds, or change it’s layer so that it is ignored then the generator understandably skips the collider altogether, I don’t get the region on top of the ceiling, but I also get an area that is too low to walk in:

You can attach a RecastMeshObj to the object and configure it to disable walkability on that surface.
Try the latest beta for an improved editor GUI for that component.