STEP files, much like any other geometry file type, represent trimmed surfaces as untrimmed base surfaces along with curves that are supposed to trim out the part that is used. This step file has as base surface, the full 360 degree sweep and some trimming curves, as shown in the file I attached in my first post. As you can see, the trimming curves are not anywhere near the surface they are supposed to trim. In Rhino4, I calculated the error in the curves and used that as the trimming tolerance. The sweep does get trimmed there and if all you want is a picture the result may be of some value. The problem is that the error tolerance in the resulting model has to be bumped up to 7 units, which is not acceptable. In Rhino5, we decided not to do that, but rather to let the trimming fail. We are working on some diagnostics to let you know when this problem happens.
I've asked @pascal for help on working around the problem. Hopefully he'll have some ideas.
Robert McNeel and Associates