I think we may have found where this is changing. It looks like the Rhino STEP import is changing the model a bit. The exported model is correct, reading back in changes it a bit. Looking at this closer.
Please check dxf too
The bug is that Rhino is changing the underlying NURBS surface geometry. The surface control points and knot structure should not be changed by exporting and importing to and from STEP
Rhino4 imports the STEP file correctly.
What Rhino is trying to do is extend the surfaces in some sort of misguided attempt to fix something that is not broken. Unfortunately Rhino is often not very good at extending (as this current example demonstrates) and also Rhino is not very good at whatever decision process is used to choose where to extend (that also appears to be what this case demonstrates)
Rhino handling of Step files has been buggered for many years. Rhino4 is the last version that is reliable. One of the problems starting with Rhino5 is that the STEP import often gets the file tolerance wrong. If you open a STEP file you will often see the model tolerance is set to an unreasonably tight tolerance and then Rhino proceeds to fix things that don’t conform to the errant tolerance setting.
All of this is poison to any Rhino user that has to receive STEP files from customers and then needs to edit the damaged geometry. For one thing its difficult to asses where the damages is. That is because (presumably) what Rhino is doing is trying to heal geometry that it decides needs healing. What that often often does is just cover up errors that don’t show up until the user tries to modify the geometry.
These efforts to heal STEP imports may sometimes be of some value to users that import geometry so that they can include it in a scene for rendering purposes. At least this was the argument made more than 10 years ago. It is sad that Rhino developers long ago gave up on making Rhino a tool for actual modeling and decided Rhino needs to just be good for visualization and have made that their priority.
Fortunately Rhino4 will never expire so any user that wants an honest read of a STEP file can use Rhino4.