It’s entirely possible the output from Rhino is not a manifold 3d object (i.e. “watertight”). Are you able to share your workspace (files → export) or object with me for analysis?
One thing I tell students is to bring their stl back into Rhino after exporting and viewing with flat shading turned on. Also run the check command on it. It won’t fix it, but it will let you know if there is a problem. If you have Rhino 8 the Shrink Wrap command is kind of magic