Preservative systems
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The preservative is the component most often chosen from a supplier's recommended usage percentage and most often the reason a product fails.
A system, not an ingredient
Preservation is water activity, the preservative blend, pH, chelators and the package acting together. The preservative is one term in that expression, and its effectiveness depends entirely on the others.
Which is why "I used it at the recommended level" is not an answer to whether a product is preserved. The recommendation was made without knowledge of your pH, your water activity or your jar.
Why pH decides whether it works
Most preservatives are active only within a pH range, and outside it they are present, listed on the label, adding cost, and doing nothing.
Organic acid preservatives in particular depend on being in their undissociated form, which is a function of pH. Move the formula a point in the wrong direction and a working system becomes a decorative one with no visible change to the product.
So pH is not a comfort-on-skin decision that happens to sit near the preservative. It is part of the preservative.
Broad spectrum means four problems
The challenge test uses five organisms spanning gram-negative and gram-positive bacteria, yeast and mold, because those are genuinely different targets. Few single preservatives handle all of them well.
Hence blends: typically something for bacteria paired with something for fungi, plus a chelator that makes both more effective by tying up the metal ions organisms use.
Choosing and then proving
Choose for the pH you intend to sit at, for solubility in the phase where the water is, and for the package you intend to use. Then prove it with a challenge test on the final formula in the final package.
And resist the instinct to fix a marginal result by increasing the dose. It costs money on every unit, it can break the emulsion by dehydrating the emulsifier, and it raises the sensitivity risk. Lowering water activity or correcting pH are usually the better repairs.