Since you have extended the post code table by the Country / Region Code field there is the question, why this didn't became a part of the primary key to be easily able to insert the same Post Code for different countries.

And currently in business central 2020 wave 2 there is an additional check on the Code field that checks that the code is not given twice in the table before you are able to input the city text! Means it throws an error before you are able to fill in the whole information. If you try to ignore that and enter the city name it throws the next error, that the field code is not allowed to be empty! So currently you are complete unable to insert the same post code twice for multiple countries or even cities.

So instead of only correct that check, you could try to get a new table into the system with code, city and country / region code in the primary key. For example as a new feature...