This is a bit different.
This is related to the Unified account rather than FBA Export.
They have added additional information into the “Build International Listings” feature to set up listings for FBA. There was a problem previously where if you had FBA, it used the inventory and pricing from one marketplace for the other. That won’t work for FBA on both marketplaces. The outcome was that if you had FBA in US and created Canadian listings, your Canadian listings would use the US FBA inventory counts, but you were not able to fulfill the orders as you needed to use FBA. That required that had separate inventory.
The recommendation was to disable the connection between the marketplaces.
That problem may have been resolved.
I used separate accounts for US and Canada to avoid that problem.
What the OP was hoping for was to be able to use FBA inventory in the US for Canada and/or Mexico.
That is not something that is available. You need to provide separate inventory to each country.
In Europe, they can allow the fulfillment to be used for different marketplaces, but that is because the EU is a federation where there is not inter-country taxes and duties. It is more like states than separate countries without customs requirements between countries.
Between US and Canada, there is different customs requirements. Amazon does not want to get involved in that for third party sellers.
After the Brexit, I expect the EFN (European Fulfillment Network) will take the UK out and require imports.