Try to evaluate the well on its own, just as a water source, and not what drives a sprinkler system. If you have to add some shut-off valves and outlet tees, consider it the cost of being able to know what you have, instead of having to guess at it.
This advice comes from the word "cavitated" that you posted, which suggests that the water source may have changed in some way. Water tables can drop lower, to a point where it is beyond the ability of a pump to bring water to the surface, at least in the same quantity and pressure as was done originally. Another change that affects pump output is a well point screen becoming clogged, with similar effects on pump output.
All of the other zones work fine though. And we’d had plenty of rain, so the water table is currently high. Even years back when we had a drought they all worked. Does that make a difference?