I just came across a bug in Nuendo v8 (same in v8.2.1 and 8.3.0):
When exporting a 7.1.4-Output and checking the option “open new project”, the playback of the exported file in the newly generated project is about -10dB lower and the Center channel is missing!
There seems to be a bug with automatic building of 7.1+4-related projects and busses. It’s not related to the export itself: The audio data is fine.
Creating a new project by hand and importing the polyphonic 7.1.4-export works flawless - the audio level is again as intended.