Amazon Canada issued buyer a full refund through A-Z claim stating I did not respond which is untrue.

The system looks to see if there has been any correspondence through Buyer Seller Messaging. If there already has been correspondence, or If you used an Amazon unintegrated tracking courier, or if the item is hazmat and the return can't be authorized in the returns center (which sounds like this is the case if the buyer messaged you about a return, instead of going through the returns center) it doesn't matter. It'll auto push through without an Amazon review first if the reason chose is defective item.

To date, I have not received the item back and the seller has been refunded through A-Z.

Use the appeal button, cite the dates and messages though buyer seller messaging that shows that the customer has received the item and it is a fraudulent A to Z claim as the customer has not provided tracking for the return.

I have had 5 a to z claims filed the last month against me. 4 of them I have won and have been denied to the customer now (3 timed out and expired as customer could not provide proof of return tracking or respond to Amazon's inquiries for more information, as they didn't actually send it back after having the return approved in the returns center, 1 was denied because the restocking fee charged was just) and I have until Monday to appeal the 5th one as the customer did actually send in the tracking info, but I didn't receive the same unit i sold the customer back complete (missing parts and a used defective return swapped out for my new one).