I bet if you fly around to the other side of the empty-looking triangle faces, they’d look filled, but the filled ones will look empty. Is that the case? If so, their normals are facing the wrong way because of drawing them clockwise vs counter clockwise or vice versa.
I stopped using a proc mesh and now am using a Hierarchical Instanced Mesh. Seems to perform quite well, not too sure if there would be much of a performance boost by trying this again.
Side note I also discovered there is a “Generate Box Mesh” blueprint for Proc Meshes. I kept typing in “Create”. No wonder it never worked.