I recall one of the Doolittle Raiders landed near Vladivostok because of some fuel problem making it certain they would not get even close to China. The Sov’s interned them for over a year, eventually sending them to a town near Iran. IIRC, there has always been some controversy as to whether the Sov’s “allowed” them to “escape” to avoid officially repatriating them.
The Soviets were neutral in our war against Japan, and under international law, they were required to intern our air crew for the duration of the war.
The whole “murky” relationship with Japan could fill a book. The Japanese were constantly egged on by Hitler to invade Siberia, but never did. The Soviets had their hands full with the Germans, as well. Because the Soviets were neutral with Japan, we re-flagged a number of merchant ships under the Soviet flag, and sent them from the west coast to Vladivostok. The lend-lease material they carried was then shipped across the Trans-Siberian railway. It made the Germans mad, but under international law there was not one damn thing they or the Japanese could do about it.