The PVB is doing it's job. You have an ongoing upstream negative pressure event occurring.
If I were onsite I would plug off all of the heads on that zone and turn the zone manually. Using a 18 " long 1" or 1-1/4" piece of pipe I'd listen to the valve and each head for water movement noise. The noise will be the loudest nearest to the compromise.
It's a pretty basic concept.
I'm wondering if you are on a well, the zone run/drawdown time is about right for a well.
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