I’m guessing this is a bug and I cannot find previous mention. When I attach a document to a purchase it tags to the receipts folder in document manager. Not sure why receipts. If I want to attach the same file to another purchase the add existing option only shows me items in the sales folder.
Any items attached to purchase invoices will go into receipts folder as they are typically purchase receipts. Right now you can only attach one item to one receipt, this would be why the item is not appearing in the search when you attempt to attach to the second purchase.