In topic Terrain WriteVoxel ignores Air material voxels
the developer reported the use of WriteVoxels evidently ignoring Air when writing to a voxel that has a non-Air material.
I see this behavior in Studio.
I modify previously (scripted) generated terrain by (a) saving the voxels, (b) rewriting the voxels with other materials, then (c) later restore the saved voxels.
The saved voxels that were all Air are not written as far as I can tell. The “Fill*” methods work (with Air).