Triangulation and vertexNormals on rotated objects

Why are there differences in the triangulation between axis-aligned cubes and slightly rotated cubes?

I have a need for normal vectors at the verticies of somewhat irregular objects (cones, rotated cubes, etc).

Issue reproduction: if I follow the examples outlined in the vertexNormal(...) documentation (vertexNormal Documentation), I get the expected results (image 1). However, slightly rotating the cube (say, 30 degrees about the x-axis--using rotx(30) or otherwise) causes the triangulation to become very skewed compared to the axis-aligned cube (image 2), specifically the top and bottom of the cube. Why? Is this a bug in the delaunayTriangulation? The obvious workaround of performing the triangulation before rotation won't work in the general cases I'm facing outside this example.