Yes you will get a full refund, We open a case for everyone one, Take a photo of the shipping label the item was returned in, The packing slip, The LPN sticker then the box and the contents, state the customer "Stole" the bags
also use the report a violation and report buyer for theft.
first you need to type that LPN into FBA customer returns to get the order number, Click the top left ladder and select, Reports, fullfillment, on the left look for Customer Concessions,fba customer returns, on that page you can enter the LPN, put date range like 90 days out
report the customer here,
https://sellercentral.amazon.com/abuse-submission/form/buyer-abuse-refund