Hi @mattm!!
First, let me welcome you to our Community! Feel free to Introduce yourself 🙆 if you feel like it 
Thanks for reporting this! I suspect you just found a bug, but I’m going to do some checks to see what is going on here, as Rhino->Rhino (and GH->GH) Breps were, to the best of my knowledge, working fine… 
I’ll do some digging on my side to see if I can offer you a quick solution, although I suspect we may need to fix this on our side.
Just as a quick note: The reason they are being displayed properly in the viewer is because the web viewer always uses the displayMesh property of any Brep/Surface, which is generated by Rhino when sending