It apen also in rhino 5, i’ts not to heavy.
Many time I imprt step file that are often divided in blocks, but not in layer, so.
-Unblock all polysurface
-Create new layer
-Select one polysurface
-Change the layer proprieties of the object to the new layer.
But at this point the new layer that I create before, are not listed. The only thing I can do, is, put the new layer as current to find the layer in the propriety list.