You are right. It seems the clustering is working correctly only on the creation plane of the Forest object. We will check the code and fix it, if possible.
If you are creating a cube, there is an alternative way to do it using XY surfaces. For each side of the cube:
1) Create a rectangle on the XY plane of the same size as the cube.
2) Add a Forest object and assign the rectangle as Area.
3) Select both the spline and Forest, and rotate them to align with the cube.
Not optimal, since it needs several FP objects, but it should work.