Russia stole some islands from Japan north of the mainland after WW2 and their ownership of them was never ratified by post war treaties. Russia offered some of the smaller ones back in exchange for Japan to acknowledge the larger ones belonged to them but Japan turned them down and maintains that they're rightfully Japanese.

With Japan building up the (formerly?) JDF and Russia very much with their hands full and their pockets empty it could be that Russia wants to demonstrate a show of force in the area to deter any attempts for Japan to take back their land.

....not that Russia could even hope to begin to support a fight off their eastern coast...but y'know...Russia and optics and shit.

Sure would be entertaining to watch them try though.