The rules on the Astral Projection spell states that anything that happens to the duplicate happens to the original as well, but it focuses on things that are lost. If a party that is exploring the astral plane happens to gain treasure, does that automatically appear on the material plane, assuming they return safely?
I don't see why not. Ultimately it is up to the GM to give out loot in the first place. They would also be the final word on whether the item has a physical form that also exists or whether the item is Astral plane only. And it could be a different answer for different items even in the same game or the same casting of Astral Projection.