You are probably exporting a concave shape. Blockland treats the contents of a LOD layer as 1 object, so even if you're exporting two meshes, BL treats it as one.
If you can't connect all of the points in an object to each other without leaving the confines of the object, then that object is concave.
So in the case of your object, you can't connect the top of the cone with the top corners of the rectangle base without leaving the confines of the shape. It's a toss up as to which points will stop working. But in this case it seems to be the point on the top of the cone which stopped working.