Hi, sorry for the slow reply.
In general I would not expect the MeshDecimator to behave correctly in this scenario, and actually both the CubicSurfaceExtractorWithNormals and MeshDecimator are removed from the development version of PolyVox. Instead we expect users to compute normals in their shader (actually we may provide some kind of normal in the future but this is still under consideration) and to use the built-in decimation of the 'extractCubicMesh()' function (which replaces the CubicSurfaceExtractor class).
Note that you can still use shader-generated normals if you have slopes, etc, the only limitation would be that you have flat-shading rather than smooth-shading.
The MeshDecimator was designed for the Marching Cubes meshes so I'm pretty sure it won't work properly for the cubic ones (or perhaps others). Perhaps you could try a dedicated mesh processing library such as
OpenMesh(I haven't used it myself)?