[Using Google Translate]
Driftsmelting will only be 2.5km if you get it after the event starts, if you get it before the event starts it will remain at 5.0km.
Did you get that stone after the event started?
As it is already mentioned by other hunters, the distance required for driftsmelting with driftstones obtained during the current event period will be reduced from the usual 5 km to 2.5 km. Please check our official website for further information.
However, please let us know if the distance is not reduced with the drifstones obtained during the event.