I have a device which is using linux gadget audio to receive audio input via USB, exposing 24 capture channels. This device works well with Mac, Windows, and Android phones. However, when sending audio from an iPhone (both USB-C iPhones and lightning iPhones using an official Apple lightning -> usb adaptor) I am seeing strange behaviour. Audio which is sent from the iPhone to any one of inputs 12, 19, 20, 21, or 22 appears in all of those channels, rather than only the channel to which audio is routed. I have confirmed on my linux device that these channels are not being erroneously populated by the software running on that device; the issue is visible in audio recorded directly from the gadget using arecord, meaning it is present in the audio being sent from the iPhone. I have confirmed that the gadget channel mask is correct for 24 channel audio (0xFFFFFF). As said above, audio routed to this device from any non-iPhone device (Mac, Windows, Android) works fine. The only sensible conclusion seems to be that the iPhone is populating the additional channels erroneously due to some bug in CoreAudio's handling of gadget audio devices. I would appreciate any insight on this from Apple developers, or from anyone else who has come across this issue and found a workaround.