I think that one problem with 3D Builder, as revealed by its so-called "wireframe" view, is that it's geared more towards .STL format, in which all faces must be triangles, hence regards non-triangular faces as "invalid", even though they're perfectly valid in other formats!
(3D Builder "corrects" my dodecahedron model to having seven(!) triangles per face, although three would have been sufficient without having to create any new vertices.) (And in any case, STL supports only one material (colour) per design, so my dodecahedron couldn't be correctly exported to.STL without losing the face detail; at least one website out there regards .STL as obsolescent if not obsolete.)