I don't have any solid experience with this board, but I just had a quick look at the manual online and came across this (see picture attached).
This reminded me of a problem I had once with a DVD player that had both anolog 6 channel out and
digital out. By default, it would only let 5.1 through the 6 channel (3 x 3.5mm stereo minijack) analog outputs, and the digital would only give out 2.0 (FL + FR).
This could be what is happening here too.
You may actually have to DISABLE 5.1 on your control panel settings (not the ones on MCE or it will downmix). This is because the board is doing the 5.1 decoding, when you actually want the receiver to be doing this.
Or you may (also?) have to check that the digital output is set to "RAW", not "bitstream/LPCM".
Don't know if this is relevant in this case, just thought I'd post my thoughts on the off-chance that it might help