I have such a problem on Magento 2.3.3 I checked this logic on stock Magento 2.3.3 and it also appears there.
Choose a 2-3 allowed countries.
Then go to All Customer -> Customer Address Edit Form in Admin area.
Country dropdown is not as we expected.
If I just use 1 country none will show up if you chose more then it will just take countries from the top.
In console i don't have error's.
Anyone had this issue ad anyone solve it?