So let’s say one turn, a pikachu eats a berry and therefore has no item. The next turn, a Pokémon holding an item with the ability Symbiosis is sent in. Does it still pass the item to the Pikachu?
Symbiosis only triggers if it's active at the moment an item is consumed. If you don't bring the Symbiosis user out until later, you're out of luck, and the item is stuck where it is.