The pH moved after filling
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The batch measured correctly, the jars were filled, and six weeks later the pH is somewhere else. On its own that is a curiosity. Given what pH controls, it is a preservation failure waiting to be discovered.
Why it matters more here than elsewhere
pH decides whether the preservative is in its active form. A drift of half a point can take a working preservation system outside its range without changing anything a customer could see.
It also affects the actives. L-ascorbic acid, for instance, degrades above roughly pH 3.5, so upward drift in a vitamin C product destroys the thing being sold.
What moves it
Ingredients continuing to react after manufacture, particularly where something was added warm. Carbon dioxide exchange in a package that is not a good barrier. Interaction with the container itself, which is why packaging compatibility is part of a stability study rather than an afterthought. And degradation products: L-ascorbic acid breaking down yields threonic and oxalic acid, which are acids, and they move the pH as they accumulate.
That last one is worth sitting with: the active degrading changes the conditions that were protecting it, which accelerates the degradation.
Measuring it properly
At a defined temperature, on the finished product rather than the water phase, with a meter rather than strips. Record it at manufacture and again at each stability timepoint, because pH is one of the attributes a stability study tracks precisely because it moves early.
A drift visible at four weeks accelerated is a drift that will happen in real time.
Specifying a range
Write the specification as a range you can hold and that the preservative tolerates throughout, not as a single number you hit on the day.
Then treat a batch outside the range as a batch to investigate rather than to adjust and ship. An out-of-range pH is the symptom; the cause is usually a raw material lot, a process change or a package interaction, and adjusting it back hides the cause without removing it.