"unknown" or not-recognised ICAOs get calculated as if they were on 0 degrees longitude, 0 degrees latitude (somewhere off the West coast of Africa, on the Equator, of course). So, WSSS may be known, the distance to any unknown ICAO, from WSSS, will result in just about 8000 nm GCD.
And "unknown" means here that the airport is not yet in the (timetable) database, so there's no GPS location known (yet). Not all 35.000 ICAO codes in the world are in there, 'cause otherwise the timetable database would become too large to handle comfortably.
A little medling with that database will fix the problem, and I'm quite sure someone of the MT will do his utmost to get the database up-to-date before the Event actually starts.
(this is one of the side-effects of creating an event that visits some very small airfields, which makes this event one of my personal favourites

)