Rather than jump to any of my own conclusions here I ran this past some people I know with actual military experience, and the response was "Yeah, those seem like perfectly normal numbers."
Could there be war crimes happening? Sure, no one is going to deny that. Killing of POWs happens in every conflict, and it would be exceptionally unsurprising in this one. The CAF - the people I spoke with - were infamous for this in WW2. But nothing about those numbers implies that in the way you're claiming.
Prisoners generally happen when positions get overrun or surrounded, but both of those scenarios are much less common than the one where your enemy falls back, generally taking their wounded with them. Mass surrenders are fairly uncommon events in modern warfare even with Russian morale as depressed as it is.
I think you're ultimately leaping to dangerous conclusions here without good reason.