I use Windows 7 in our Mix Theatre and after upgrading to Nuendo 8.1.10, the 5.1 output renders all 6 channels, but the only tracks that have sound are the 1st two. Left and Right. I opened the same project in Nuendo 7 and had no issues with the output. My Windows 10 machine had no issues with the new N8 version, so it seems like it might be a Windows 7 issue.
I just rolled back to Nuendo 8.1 and the problem is gone. It seems to be 8.1.10 and Windows 7.