The pack and the formula reacted
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A formula that was flawless in development can fail in the tube, pump or jar it ships in. The container is a participant, not a wrapper.
A product is a formula in a container
Compatibility testing exists because a product that is perfect in a laboratory beaker can react with the actual packaging it is sold in. The formula and the pack are one system, and a stability study is only meaningful when run in the intended packaging.
What actually happens
Absorption. The pack takes something out of the formula. Fragrance components and preservatives are absorbed into some plastics, which quietly reduces the level of the thing protecting your product.
Migration. Something comes out of the pack and into the formula: plasticizers, residues, or colorants.
Permeation. Oxygen and moisture pass through the wall. A thin plastic bottle is not the barrier glass is, and over a year that difference shows as oxidation and as pH drift.
Attack. The formula degrades the pack. Low pH products and some solvents craze plastic, corrode metal components in pumps, or degrade the gasket.
Mechanical. A pump that will not lift a thickened emulsion, or a dropper that will not release a thin one.
Why a beaker tells you nothing
None of the above can happen in glassware. Every one of them requires the real pack, the real closure and the real liner, filled the way production will fill it, and stored long enough for the slow mechanisms to show.
Which is also why a packaging change is a retest rather than a purchasing decision.
Testing it
Fill the intended pack with the final formula and run it through the standard conditions: accelerated at 40 and 45 °C for four and eight weeks, freeze-thaw between −10 and +25 °C, and real time at 25 °C and 60% relative humidity.
Track appearance, odor, pH, viscosity, actives and microbiological quality, and also look at the pack: weight loss through the wall, discoloration, a pump that has stiffened, a liner that has swelled. The container is under test too.