As described in the manual here: http://www.volumesoffun.com/polyvox/documentation/0.2.1/manual/LevelOfDetail.html#cubic-meshes
A correct solution would be to provide a simplification function which does not create these T-Junctions. However, this will almost certainly result in more triangles and vertices and may also be more computationally expensive. Maybe it should be provided as an option.
Another possibility would be to fix the holes with a post-process (medial filter combined with triangle edge detector?). The feasibility of this needs investigating.