Return returned to FC but not to my inventory

Hi,

just a small problem, I sell FBA and customer has been refunded and returned an item to FC (according to Seller support), but as the item is defective it was not reimbursed however neither it has been returned to my stock as unsellable for removal. I keep asking Seller support to identify where the item is so I can remove it and have it replaced under warranty but I do not seem to be able to find the returned item anywhere and SS keep sending me the same response saying it has been returned to FC by customer.

Any suggestions how to find the item or prove Amazon that they lost it and reimburse me?

Many thanks