Hello,
what happen indeed is that the real surface of the solid black parts could not be recovered. This is not really surprising, because in general, photogrammetry methods need nice surface texture in order to work as it needs to visually identify the same points in more images. Finding which “black pixel in a spot” is which “black pixel” in an other spot is impossible even for human. In case of solid colors, there always remain at least one dimensional ambiguity, as geometrical constraints can be used too.
That means, there are more interpretations of the same 3D model (more models projecting to the same looking images as the provided ones). RC returned one of them.
What can help in such case is adding texture (e.g. using powder, projector, lamp,…)