At this location, If you are somebody local or somebody that knows the situation well please verify whether the relation Yuzhno-Kurilsky District https://www.openstreetmap.org/relation/1778093 has the correct extent. According to https://www.gsi.go.jp/kankyochiri/gm_japan_e.html Japan also claimes Iturup island.

Additionally, a suggestion is to tag the boundary of

Iturup - https://www.openstreetmap.org/relation/2103185
Kunashir - https://www.openstreetmap.org/relation/2409701
and other islands that are included in Yuzhno-Kurilsky District https://www.openstreetmap.org/relation/1778093

as disputed (disputed=yes), because the dispute is still ongoing per the territorial disputes

https://en.wikipedia.org/wiki/List_of_territorial_disputes