I can think of a good reason for Japan and Russia to not be at war.
For Russia: not having to deal with slowly losing IPCs and having to think about diverting nessecary offensive units to a far off and unimportant front that it will have very little chance of gaining the upper hand in, espically if japan diverts a chunk of its airforce there.
Japan: There is no victory city objective in the Soviet Union that the Japanese can realisticly capture, which it needs to win the game. They will be making thing more difficult for their Euro-axis allies by giving the Soviets and income boost from round 1 greater then the amount they can take from the soviets right away. Moving offensive units away from more vital fronts in the south to play wack-a-mole with soviet infantry with a minimal and slow economic gain for Japan.
The Japanese stratgey of “back dooring” the soviet union is no longer useful. In order to win the game, the axis powers must capture 14 victory cities. If Japan sends forces against the Soviets, it is moving units away from the nessecary 4 or more victory cities they need to take to secure an axis victory. Even if Japanese forces in siberia help the Euro-axis take down the soviet union, that will still olny give them 3 victory cities between them giving them a total of 9 cities, 5 short of the 14 they need. If Japan goes south, it can capture at least 3 easily, which will help the axis more towards a global victory.