Interesting suggestion! On its own it didn’t work, but I tried just deleting the Stereo Out and re-adding a new one. I’d avoided doing that - it seemed a bit like the Nuclear option - but it worked like a charm. Now I can see Left/Right. I just had to re-designate it as the Main Out and do some re-setting-up in Audio Connections.
The Stereo Out is now at the bottom of the list instead of the top, which will take some getting used to, but at least I know how to deal with that problem in the future.