Hi all!
it looks like multiple UV regions to me. Zbrush can’t create Normals when your mesh has multiple UV regions.
a quick question… did u unwrap your model after completing modelling, and then imported the unwrapped model back on top of sub1 ?
If thats the case try going up to highest subd in that subtool, and see if the mesh still holds… cause to me it looks like a mesh explosion…