Unforecasted versus unexpected

Solution 1:

I'd go for 'unprojected', 'unanticipated' or 'unforeseen'. Although 'unexpected' may be best.

NB: 'Parties short longevity' means parties that have a financial short-position in (or short-exposure to) longevity. They are effectively betting on longevity decreasing (perhaps for sub-populations), but were then wrong-footed when, for example, HIV/AIDS-medicine was developed. Or pension funds. It's finance lingo.

Solution 2:

"Unforecasted" works with budgets and forecast, where you are trying to indicate that something was not included in the forecast. for instance.

The covid pandemic has resulted in an unexpected increase in demand for read to eat products.

However if it should have been known,

The April budget for the new year did not include an unforecasted increase in ready to products.