For store_availabilities the liquid documentation says “If a location doesn’t stock a variant, then there won’t be a store_availability for that variant and location. The store_availability object is defined only if one or more locations has local pickup enabled.”
In my case, the store has locations that meets both requirements - there are locations with local pickup enabled and those locations do stock the variant (“stocking” a variant is not the same as having the variant currently in stock). But even with those two requirements met, the store_availabilities array is empty unless I manually adjust the stock quantity from 0 to 1, and back to 0.