Weird normals problem, not flipped

I’ve been working on this dude, all went fine, until I exported to OBJ and loaded in to Maya. One piece of geometry has messed up normals. They aren’t just flipped, because flipping them doesn’t do anything, I have to get Maya to recalculate them from the faces.

It’s odd that the geo looks fine in Nomad, and that makes it difficult to work out what has gone wrong. Even if I remesh the object the problem persists – I’ve tried Nomad’s quad remesher, Exoside’s quad remesher, voxel remeshing, and decimating, all to no avail.

Does anyone know how this has happened and how I can avoid it in future? Any way to fix it in Nomad?



Bit more info – according to Substance Painter the normals are null