Same here. As i see, Upwork does not provide your client's ID Number. So the "invoice" they provide from the client isn't a valid one. Has the same value as a line in your notebook. Then i think you have to make a valid invoice. The question is who to "invoice" the client or Upwork. I think you have to invoice Upwork because its' the one managing the money. They make invoice for you and invoice for the client. In the other hand, Upwork fee invoices are REAL invoices with all the right details ain ID numbers, that are expenses for you.
Im sure that the important thing is that you declare the amounts. But I would be happy if i get a more clear explanation.