Inauthentic Claim on Items Straight out of Casepack - Suspect Buyer Swap

Got an inauthentic claim on an FBA order from a customer. All of these were straight out of manufacturer case packs. I could just upload invoices and move on, but I'm irritated that I get accused of this when I've sold a million things, literally, and never once would have dreamed of selling a fake, so I tried to get the exact reason out of the seller performance team, but they won't tell you. I have one other return for the product, but stated reason is did not arrive on time. Account Health team did tell me complaint was recent, and there is only one recent order, so I tried asking the customer if there were any problems with the order. The responded that they received what looked like an authentic item, but when they opened the item, the stuff was fake inside. I suspect the customer who returned the item a few weeks ago saying it didn't arrive on time just swapped it for a fake because Amazon sent it back to inventory and then this customer claims they got a fake. Any good way to go about this short of sending invoices and moving on? So sick of this kind of fraud and then getting called out on the carpet over it. I'd like to at least try to bust up the bad actors even if nothing comes of it. The last inauthentic complaint I got turned out to be that the customer got X style instead of Y style on a commingled unit, but somehow Account Health marked the situation as an inauthentic, so forgive me for being skeptical of the whole process. I just really don't like the feeling of being falsely accused.