@SkippyBing - Part of the testing process will involve flying the plane around various parts of the world to see what sorts of problems will crop up. They won't be looking for just problems with the plane itself, but also problems with the training, base facilities, communications systems, spare parts supply and inventory system, ground equipment, etc., etc.
The plane software is also localised, in terms of having to download new software components to support features in specific parts of the world. The details of this are not public, but apparently if you buy the plane for use in one part of the world you have to get a software update from the US to use it in another part of the world. The issue is apparently related to limited data storage which prevents them from downloading a global data set to all the planes.
The end result is that they can't know if the plane, including all its support infrastructure, is going to work in Korea until they actually test it in Korea. The presence of the F-35s there is only making the news because of all the other things happening are causing the press to report whatever other military news they can find.