The AirTags location is only as good as whoever's iPhone, iPad, or Mac they ping off of. If the AirTag is in the luggage hold and someone on board has forgotten to turn their phone to airplane mode, the AirTag will use the phone's location, but the phone on board might be confused since it can't get a good GPS fix or cell service, and therefore reports its last known location.

I bet something like that is what's going on. I think that once the AirTag has a chance to ping off of a device that knows its location in Seoul it would work fine.