Hi!
good to see that i am not the only only one who has this problem.
i tried the multichanel capture with a sony pdw f70 XDCamHD deck, same result as you.
so, perhaps it is not a HD problem since you are working in SD.
however, i can send multichannel audio to the deck via HDSDI without problems.
if you or anyone finds a solution, please let me know.
next i will try to contact the decklink support, maybe they know what we are doing wrong (or not)
greetings, michael