Polygons are single sided by default, which saves computing power because the software doesn’t have to calculate data that isn’t seen by the viewer due to being the inside of the mesh. However in your case, the normals (facing direction) of some polygons are flipped for some reason.
The easy solution to this is Tool >> Display Properties >> Double, which tells ZBrush to render both sides of all polygons.
A harder solution is to hide the parts of the model that are correct, leaving only the flipped normals. You can then use Tool >> Display Properties >> Flip to switch the direction of the visible polygons. This way, all of the polygons will be aimed outward and you won’t have the issue anymore.