Pulseaudio clients utilizes aux channels
There was a trick in pulseaudio that allowed to split multichannel USB audio device into multiple stereo and mono devices by creating mappings where unused channels are mapped to aux* channels. See native-instruments-komplete-audio6.conf for example. However this seems doesn't work with PipeWire. Pulse clients are still utilizing all available channels for some reason. I'm not sure if this is intended or a bug.
Attaching my ACP profile just in case.
pulseaudio client libs: 13.0 and 14.99.2_pre1