Hunter PGV diaphragms tearing
I discovered that there was a part missing from the valves that kept blowing diaphragms. There is a white support ring that sits inside the valve body, under the diaphragm. On the two zones that kept failing open, it must have stuck to the diaphragm's plunger piece when it was first replaced. After replacing the missing piece (Hunter part number 331300), I haven't had a problem since.

FWIW, Hunter and their local distributor were worthless for sorting this out. Hunter blew me off with a talk to a dealer response, and the dealer said the part wasn't available separately from the valve. The support ring is available online, and is much cheaper than whole valves, even after shipping.