What does actually happen to a 7.1 LPCM stream when the 861 is set for 5.1 and you have no control to down mix from the source? Does the HD621 disregard the two channels, down mix 7.1 to 5.1?

Living Room Kit: 861v4+ID40, DSP7000, DSP5500, DSP5500HC, HD621
Bedroom Kit: F80
QNAP TVS-471, Roon, iPad Air2