Figured it out ...
I'm posting my results here just in case someone in the future runs into the same exact problem.
The macroblocking I was seeing on KPRC 2.1 OTA only occurred when it was being played back on an nVidia GPU. When I viewed the exact same recording, from the same .WTV file, on an AMD GPU, the macroblocking was gone.
I reverted to old drivers, tried other nVidia cards, fresh copy of Windows 7 (as I use Media Center), etc, etc ... always the same results with OTA 2.1. This only started happening a couple of months ago, so I suspect KPRC changed something with their broadcast codec or compression algorithm that nVidia just doesn't like when it is stored in a .WTV file format.
So, I ditched the GTX 1060 in favor of an RX 570, and the problem is GONZO, hopefully for good.