there appears to be a bug in Rhino 6 that will prevent a curve (object) from being properly displayed once a section cut ( mirrored clipping planes) are turned on for a model. The cplane is set to a surface and there is sufficient space on either side of the surface. The object is drawn with project on so it will only draw on the cplane. Once drawn the item is not visible but is selectable and dashed when selected. Once the clipping planes are turned off, the object is clearly shown again on the cplane.