I see my problem is similar to Roon confusing my computer’s audio devices with core’s audio devices
That was solved by re-installing windows - which I will not do (nor re-install Mac OS).
The Mac mini core has had Mac OS on it for a while (with os upgrades) AND had a Roon client that I have removed since installing Roon core. The Mac studio is a few weeks old and I used the Migration assistant to move apps over.
I just reconnected the Mac Studio to a core on the Synology and lo and behold - the audio devices under the “Mac Studio” are correct- I can see the DAC.
So @support - it appears even if I remove every last trace of Roon files on the Mac mini and Mac studio and re-install - it still is confusing audio devices between the Mac running the client with the ones on the core. So something is left that I didn’t remove - OR - you’re storing something in the “cloud” that is confused about which Mac (my core or client) is looking to play music.