Related to that, if there is delamination, then the same can happen inside the board. Especially between inner conductors (since, thru holes have outer layer annular rings closer together than the holes).
Even without delamination, I would also be curious what the lateral breakdown is.
There's no particular reason to expect it to be isotropic -- the fiberglass weave is not evenly distributed, and is oriented in plane. There may also be gaps, voids, defects in the layup at edges of inner layer copper -- the edges are mechanical displacement, stress raisers and concentrate the electric field.
Tim