Presumably this is calculated into the contract.

If the price is expected to fall over time, then the negotiated price is below market at the beginning and above market at the end. From the suppliers view, they take a loss in the early years that they recover later.

Apple probably pays a premium to shift this risk to the supplier. Besides that they don't take a loss just because prices tend to fall. They only lose if market prices fall more steeply than expected.