https://en.wikipedia.org/wiki/Unit_731#Separate_Soviet_trials

The sentences doled out to the Japanese perpetrators were unusually lenient by Soviet standards, and all of the defendants returned to Japan by 1956.[111]

In addition to the accusations of propaganda, the US also asserted that the trials served as a distraction from the Soviet treatment of several hundred thousand Japanese prisoners of war; meanwhile, the USSR asserted that the US had given the Japanese diplomatic leniency in exchange for information about their human experimentation. However, it is likely that former Unit 731 members had also passed information about their biological experimentation to the Soviet government in exchange for judicial leniency.[111] The Soviet Union built a biological weapons facility in Sverdlovsk using documentation captured from Unit 731 in Manchuria.[112]