OK – it seems to work fine.
I'm feeding the H8000FW to/from a UA Apollo 8, using the factory "ADAT A->B" routing on internal 96K clock.. I'm running "Oscillator 440" on dspA and "Thru" on dspB.
The 4 ADAT input channels come in on channels 1,2,3,4 and are mirrored on 5,6,7,8. This is shown on the input meters.
The 4 ADAT output channels are fed from 5,6,7,8. This can be proven by varying the digital output volumes.
I'll accept that the mirrored input channels may be confusing, but it is probably out of a desire to avoid "empty air."
Similarly, the use of the higher output channels is odd, but presumably there were reasons.- probably to keep the analog outputs free.
If you are having S-MUX trouble, and have the time and interest, please set up your unit as above and tell me in detail what you see.