All sites that we've visited now have the "Y" power cable, generally providing 38+VDC to the Victron.
There was an issue at s9, I believe, that one of the lower panel's connectors was very corroded. I only got 30VDC in full sun with the Y cable installed. However, I then cleaned this corrosion as much as I could with a stick and got up to at least 36V, so I assume that the corrosion had started shorting this panel.
Also, s4 was connected with a Y cable when we got there, but in a configuration which (I think) would have ignored one of the panels. It should be happy now.