The record derives from WCSP (in review) (data supplied on ) which reports it as a synonym with original publication details: Res. Bull. Coll. Exp. Forest. Coll. Agric. Hokkaido Imp. Univ. 7: 190 1932.
This name record (from WCSP (in review)) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Polygonum sachalinense in version 1 of The Plant List.