You are very welcome, I would rather look at these issues than ignore them, as the ZS is a fantastic piece of gear for the money, and simple sometimes "possibly" overlooked issues like this can cause great heartaches for those of us that have many pieces of gear or like different DACs for different things. Yes, you can put me in the "rare" category as well, I hear differences! (Spoken like "the sixth sense"

)
So if you are willing to play along, here is what I did that seems to have "temporarily" eliminated the issue prior to yours and may help you as well. I am not having the issue with Spdif selection since I did this, and I am in AIO mode for
@yoghurtlidlicker issue with USB drive connection, but it did help with my Spdif connection to my "Completely Non-Conforming AD1865 NOS R2R Spdif only Chi-Fi Wonder DAC", so hopefully it will help you out.
From inside the Stream-ifi app or ifi.local, either is fine, do a soft factory reset, wait until it is completely finished, and then update to the current firmware!
Once this is done and functional (test anything you'd like) update to the latest Beta 2.31.10 and see if your issue is resolved.
If this does not help, you can always go back to the current firmware if it gives you any grief.
Quickly on the subject of the Signature V2 working better, this is not one bit surprising to me as it should, in fact, I have never had a piece of iFi gear that didn't work hand in hand with another. It's a great big world of tech out there and obviously, ifi has access to their own gear, and not every other manufacturer. In that case it takes folks like us to point out what is and is not working "hand-in-hand".
Cheers!!
A pic of my Nonconforming DAC, lol