I checked PA 15.0 changes, but I couldn't find any relevant change to the problem. And, as I tested PA 15.0 on Leap 15.3 and plug a USB audio device, the IEC958 shows up in the profile. So I don't know what went south on your system. It looks like that things should work. Maybe some conflict in the saved configurations? You can try removing the entries in ~/.config/pulse/*.