The Russians and Japanese had been in conflict on and off directly and by proxy since early 1900s. In the 1930s when the Japanese were heading West through China toward the Soviet border, it came to a head but both empires were distracted on other fronts for a full commitment of resources.
Battle of Lake Khasan took it to WW2 levels before a formal declaration of war.
Yes, I am aware of that - but it still bothers me when reference is made to "Soviet POWs in Japanese hands" without any further explication.
These Soviet prisoners were not POWs in the "classic" sense of the word. They were military personnel engaged in skirmishes, border incursions, undeclared conflicts, and the like who were kidnapped or otherwise fell into enemy hands and were interned, but who were probably never properly recognized as legitimate POWs under the rules of war.
Regards,