We are selling through the Remote Fulfillment option, but customers are seeing the option to buy from MFB (Merchant Fulfilled)

We have been selling on Amazon Mexico and CA via Remote Fulfillment using US inventory, and we've been doing so for a couple of months. However, since July 1st, we have noticed that customers have been given the option to choose fulfillment by us and Amazon FBA. We don't understand how this is possible without having merchant inventory in this location, and we have received a couple of orders from Amazon.com.mx.

Seller Support says we need to upload merchant inventory as zero units, but my question is how? There are no ASINs available in the merchant inventory channel.

I am so lost here. Please help with what can be done