Well, kind of solved my own issues.
Apparently the zone that was “bad” actually had a bad solenoid in the new valve. I had another new valve so thought what the heck, I’ll try connecting just the solenoid to the zone and give it a go. Solenoid buzzed right away when activating the zone from OSPi so I screwed the new new solenoid into the valve and before firing it up went to check the voltage on the zone. Zero volts AC in that zone now! Just for fun I checked the other unused zones and they still show ~7VAC but the one I care about seems to be back in order so I used the app to turn on the bad zone and all is happy again. I tried the zone four or five times then let the normal program run yesterday and all seems well.
Not sure why the unused zones are showing ~7VAC but I don’t have anything hooked up to them so not going to worry about it for now.