Hmm, good question. When Grasshopper copies or saves data into the clipboard, it uses the oldest version of OpenNurbs it can get away with. In this case probably Rhino5-style opennurbs data. That flavour didn't support n-gons, so that information gets stripped.
If it saved Rhino6-style opennurbs data then you couldn't paste that data into a Rhino5 instance running Grasshopper. I concede that is probably a fairly rare case, so maybe it makes more sense to always use the current version of Rhino when creating clipboard data.