So I played a game where as I was USSR, US decided to bomb a Japanese transport and land it in USSR territory as well as asking me to leave infantry behind to defend the bomber if Japan goes after it.
Historically, there were a few cases of US B29s bombing Japan and then, due to battle damage, flying onward to land in the relatively near USSR rather than returning to their home bases. The planes and their crews were interned by the Soviets because it was a violation of neutrality for the US to use the territory of the USSR as part of a military operation against Japan, given that the USSR and Japan were not at war with each other until August 1945. The OOB Global rules don’t go into that level of detail, but they do say:
Due to its separate treaties with Germany and Japan, the Soviet Union is in a unique position in its relationship with the Axis powers. As a result, if the Soviet Union is at war with Axis powers on only one map, it is still under the restrictions of being a neutral power (see “Powers Not at War with One Another,” page 15) on the other map. In other words, a state of war with Japan lifts those restrictions from the Soviet Union on the Pacific map only, and a state of war with Germany and/or Italy lifts those restrictions on the Europe map only.
From my understanding of OOB Lend Lease, it states that it is broken by Axis movements or any allies in said original USSR territory which was broken for me even though said units were not being used against Germany or Italy.