You make a couple of good points, Web.
It's possible, according to your description of Soviet defensive strategy in the early stages of the European Twilight War, that on a couple of occasions at least, Soviet defense belts were penetrated and Soviet/WTO formations encircled.
This would justify the large numbers of captured, viable Soviet AFVs. I was trying to help you out!
As a historical sidenote, the Korsun-Cherkassy operation was conceived entirely as an encirclement operation from the get-go and was approved by STAVKA/Stalin as such. He was rather keen on repeating the success of the Stalingrad encirclement. Despite early successes in severing the German salient in the Dnieper bend, the Soviets did not continue to feed units into the holes they punched in the German lines, instead being perfectly content to settle for a more modest double envelopment and pocket reduction operation. Envelopment operations were a key component of Soviet Deep Battle doctrine. Perhaps it was not the goal in all cases, but it was a standard operational contingency.