To make sure we are on the same page:
1. when measuring voltage, you should be using your multimeter to measure AC voltage (not DC) from the COM (common) terminal to a zone port. You should NOT measure voltage from GND to a zone port as that’s not how the solenoid driver circuit works.
2. if you remove the zone wires (or simply remove the common wire), and measure voltage as step 1 describes, and turn on each zone one after another. When a zone is turned on, it should measure about 24VAC. When a zone is turned off, the voltage may float (not 24VAC but also not 0), that is normal.