pH, and what it decides
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pH is the single number that touches more of this craft than any other, and it is adjusted at the end of the batch almost as an afterthought.
Four things at once
It decides whether the preservative is in its active form. It decides whether the actives survive and function. It affects emulsion behavior, because emulsifiers have ranges too. And it decides how the product feels and whether it irritates.
Four constraints on one number, and they do not always point the same way.
The skin argument
Healthy skin sits acidic, and a product far from that range disturbs the barrier it is supposed to support. That is the case for formulating near skin pH for anything left on.
But it is one constraint among four, and the mistake is to treat it as the only one and then wonder why a preservative stopped working.
The stability argument
This is where the conflict becomes concrete. L-ascorbic acid is at optimum stability around pH 2.5 to 3.0 and degrades above roughly pH 3.5, along with degrading in oxygen, light and heat.
So an effective L-ascorbic acid serum sits at a pH that is low for skin, hostile to many preservatives, and demanding of the package. That is not a flaw in anybody's formula; it is the actual constraint, and the products that handle it do so by using a stable derivative, or by accepting a short period after opening, or by using airless packaging in dark glass.
Measuring and adjusting
Measure the finished product at its final temperature, not the water phase while hot. Use a meter rather than strips, because the resolution you need is finer than a color change.
Adjust after everything else is in and the batch is cool, record the value in the batch log, and set a range rather than a target, because a range is what you can actually hold and it is what a specification should say.