For example, Player A is originally from Japan. His flag changes to the US within 10 minutes. Reverts it back within another 10 minutes.

If you use logic and check flight times, a non-stop flight from Japan to the United States is about 13 hours. A round trip altogether is about 26 hours. If player A managed to “travel” to two countries within 20 minutes, you can conclude something is suspicious. Either way, it shouldn’t be allowed when it comes to in-game activities.