Do Boxed Mashed Potatoes Ever Really Go Bad? - Tasting Table
Briefly

While the package remains unopened and intact, your instant mashed potatoes should still be good, even well beyond the best-before date.
Although the dry environment generally inhibits the growth of bacteria, any breakdown in the packaging or changes in temperature can allow moisture to enter which can lead to mold and the growth of bacteria.
Read at Tasting Table
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