Unusual internal cavity / inverted faces when visualising StructuredQuadBox mesh

[Belay-129]

Hi Uw3 team,
When creating and visualizing a simple StructuredQuadBox mesh, the rendered mesh appears to contain an internal hollow region / inverted face, giving the impression of a missing or collapsed interior. This is unexpected for a uniform structured mesh and makes it difficult to visually verify mesh integrity.

The issue appears during visualization rather than mesh construction logic, but it is unclear whether this is a rendering artefact or a deeper mesh connectivity issue.

" import underworld3 as uw
from underworld3 import visualisation as vis

mesh = uw.meshing.StructuredQuadBox(
minCoords=(0.0, 0.0, 0.0),
maxCoords=(1.0, 1.0, 1.0),
elementRes=(20, 20, 20),
)

vis.plot_mesh(mesh)
"