I heard conflicting statements about this topic. Some say the Soviet Union broke its non-aggression-pact with Japan to conquer parts of Manchuria/China/Korea while others say that attack was a lie the Japanese government used to “justify” the unconditional surrender to the US.
It definitely wasn't made up by anyone, and I have actually never before heard anyone denying the Soviet invasion of Manchuria so this is a new one.
Following the Tehran conference in November 1943, Premier Joseph Stalin had agreed that the Soviet Union would enter the war against Japan once the Germans were defeated. At the Yalta conference in February 1945, Stalin consented to join the Pacific conflict against Japan within three months of the war’s end in Europe.
On August 8 1945, the Soviet Union declared war on the Japanese Empire. This declaration was made by Soviet Foreign Prime Minister Vyacheslav Molotov to Japanese ambassador Naotake Satō at 11pm Trans-Baikal time. An hour later, the Soviets began their advance simultaneously on three fronts: to the east, west, and north of Manchuria.
By September some Soviet forces had made some landings in the Kurile Islands, but their relative inexperience in amphibious warfare, combined with the usual spirited Japanese resistance, limited their progress before all armed forces stood down. Behind his promise to the Allies, of course, Stalin had hoped to make some inroads into the Far East and got what he wanted - among other things, payback by retaking Port Arthur and the establishment of a pro-Soviet regime in North Korea.