I believe this is a Roon/Kernel issue.
There is a similar thread on this here; where I have been quite active.
Later kernels (Like Beta Ubuntu 21.10, Kernel 5,14 I believe) fixed the unused channel issue for my Motu MK5. This was with a direct USB connection to my Roon server.
However If I moved back to an endpoint strategy using a ubuntu/roon RPI4 bridge endpoint (Running Kernel 5.11 for instance) the noise returned on those 9-10 channels. In my case I believe this has something to do with the way Roon addresses unused channels (I.e. over 8). Not sure how it would apply to a stereo situation.
In the end I proved the Motu MK5 works fine on the later Kernels, with direct USB connection to the Roon server.
However in the end I actually went back to Roon Rock (for my server), and RPI4/Ubuntu on the endpoint and simply muted channel 9-10. But its good to know Ubuntu 21.10 server beta was perfect.
I’ll make the jump to 21.10 (Kernel 5.15) fully on both server and RPI endpoint once its officially released on the 14th of Oct.
I understand you are using a stereo 2 channel setup, but the issues are similar.