It's worth noting that these tests aren't completely comprehensive. Recall that the nominal shelf life of various Covid vaccines was extended a couple of times - the initial results were interpreted conservatively, but over time more evidence allowed a more confident prediction of a longer shelf life.

When it comes to a vaccine that's not expected to be useful more than ~6 months into the future (nobody's taking the flu jab in the spring, and next year they'll want the new one) there's not really much point in measuring how it lasts beyond that with any degree of rigour.

Providing the shelf life is expected to be good enough for "this year's flu season" (3 months?) they'll most likely just use that figure and move on to more valuable work.