I have just one country allowed in configurations. If I try to create new address for customer in admin, the country dropdown does not show any countries, not even one. I can't save the address, because country is required selection.
If I allow let's say 10 countries in configurations, it now shows 10 countries, but not the ones I allowed in configurations. It shows first ten countries from Magento's country list.
In customer account in frontend the correct countries are shown and it even works with only one country allowed in configurations.