If you have an Amazon account that sells across multiple countries, such as United States, Mexico, and Canada, then you have encountered regional inventory syncing.
When Listing Mirror syncs inventory to SKU ABC for Amazon US, if you also have SKU ABC in Amazon MX and CA, then Amazon will take whatever quantity we sent to SKU ABC for US, and push said quantity to MX and CA. Listing Mirror is unable to prevent this.
This can be exceptionally frustrating, if SKU ABC is an FBM SKU for US, but an FBA SKU for Amazon MX. This regional inventory syncing will cause the FBA SKU on Amazon MX to flip back to FBM, and then that FBA QTY will be stranded.
If you are running into an issue, you will need to ensure your SKUs DIFFER across your Amazon regional accounts, when differentiating between FBA and FBM. As long as the SKUs do not match, regional inventory syncing will not occur. So using our above example, changing SKU ABC on Amazon MX to SKU ABC-FBA will resolve this issue of SKU ABC being flipped to an FBM SKU in Amazon MX.