Flight JA818A-1482136116-adhoc-0 has bad/undocumented data! For FlightInfoEx, the “origin” field contains “L 35.54851 139.83610” instead of an ICAO/IATA code.
I see this must be a very special case, as the fa code has “adhoc” in the name. Still, it is undocumented data.
Even IFR flights are not legally required to always start or end at an airport code–they can also involve latitude/longitude positions (“L xxx yyyy”) or at a radial from a navaid (“R xxx yyy zzz”). However, if you enable visibility of position-only flights via your account’s settings on the website, you will be more likely to see flights that begin or end at a latitude/longitude position, representing where FlightAware detected the flight.