Problem with simple fillets

The attached image is my Grasshopper layout for a rectangular box that has filleted corners. Needless to say the box looks A-OK in Rhino - see box at top right. But when I bake & export this as an STL file with 0.005 tolerance (dimensions are set to millimeters) and then import that same STL file back into Rhino I get the box at lower right

Clearly there is something wrong with the fillet surfaces. Is this bug in Grasshopper, in Rhino, or am I doing something wrong or missing some steps?