Absolutely no thermal drift with EPS, from the time of molding to end-of-life.
You must be thinking of
isocyanurate foam, which is used in many SIPs. This has rather higher insulative value than EPS when first manufactured because inside the foam cells is 100% blowing agent, which is heavy slow molecules (usually refrigerant gasses - ozone destroyers) that do not conduct heat well. But from the moment it is blown, iso foam is offgassing its nasty chemicals and dropping in insulative value fast.
Of course customers are told the insulative value as tested at time of manufacture, and are not told of thermal drift. And there's no one to keep them honest.
Understand the reason this drift happens is because of the exchange of heavy molocules for air. As insulation, iso and EPS foam both hold pockets of isolated air which greatly impedes heat transfer, so they have much the same mechanism.
EPS is blown with steam and is filled with air from the beginning, so there is no thermal drift over time unless it is exposed to ultraviolet light which degrades it. EPS can absorb water intracellularly, but only if submerged for 24 hours (<2.75%), and it contains essentially 0% when part of a wall system. As the cement hydrates it will draw any water it can.
I could look it up, but will instead risk sounding like a fanboi: EPS should last 1,000 years, it is totally inert, and it exhibits no offgassing. If you find different, we would like to hear about it here.