Output is only in mono, not stereo in Mainstage 3

Ah, that's progress. Have you any other apps that can play stereo and be panned ? If so, try one.

Also, check the "Sound" section of the "System Preferences" to see if you have a balance control showing when you select the sound output device you are driving from MS.

You could also check what's set in the "Audio MIDI Setup" (access from the "Other" app group in Launcher or directly from the Applications folder in Finder). Go to the Audio window and check that the sound device you are using is set to at least 2 channels.

Hopefully something else will show up with one of the above! Good Luck!

I assume the Steinberg UR22-C is a USB connected audio output device based on a quick google search. That being the case, have you tried using your default audio output to see if the problem still exists? If not, it might be that the driver for the Steinberg UR22-C needs updated?

All I can suggest is to try a totally new MS concert with the simplest of instruments and the MS Audio output set to the default audio device; I have a template in mine "keyboard minimalist"; perhaps just try that and see if you still have the same issue.

OK, last try from me. Have you any software installed such as LoopBack or Audio Hijack (both from Rogue Amoeba but there are other similar apps from other authors, SoundFlower was a freeware app that could also do this) that can intercept the output of an app and route it? It seems from what you've written that MS is by-passing the usual route and getting directly to the output audio device. It might be worth trying a demo of one of those to see if it throws up any clues. Sorry, out of ideas otherwise.

On the channel strip that has the instrument you are playing, what is set in the "Output" drop down and also is anything set in the "Sends" drop down?

Hmm, don't know. I'm on Monterey 12.1 MS3.5.3 on MacMini M1 and it pans as it should.

Are you panning on the instrument channel strip?

When you look at the green vertical sound bars on the channel strip indicating left and right volume, do they stay the same or do they correctly show the panning?