The reason I asked about the direction of the valve was concerning diaphragm valves. They will use water flow to help hold them open. In your case you are correct this is not the issue you are describing.
With a measured 24VAC output on your transformer you do not have any DC issue. If you did it would be very apparent in the output measurements.
The only thing I can think of at this point is that perhaps you are running into the power output limit of the OS. Though this should keep the valve from opening, not closing…
Next question would be why are you able to measure 24 VAC when the valve is supposed to be closed? This is probably best answered by Ray but I will look into the hardware for 2.3 to see if I can find something. Also check your common wire vs the ground of the power supply. It should measure 0V, if not you may have a grounding issue, but again this is unlikely as the cause.