If you have no control to down mix, and the auto detect EDID information is ignored by source, then I believe that the additional channels are simply ignored. Ie you get a non optimal filtered 5.1 subset of a 7.1 source.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).