An Observation on Mystery Heat Flow

I couldn't figure out why one zone's supply was warmer than the other when both zones were on while running off the tank.
It seems that the 1/2" makeup water piping I added to the return side of the boiler protection valve for easier system filling is the culprit. Return water flowed around the valve, up the bypass line and to the supply. When I turn that valve off, voila, the zone supply temperatures equalized, and when I turn it back on it's back to badness.

I'm tickled that I was able to solve the issue with no cutting of pipe!

Yes. In my terminology makeup water=fill water. No new water is being admitted, but the small line I added around the thermomix valve allowed flow around the valve.