Chuck Cota

I have a 2021 Silverado High Country with the 6.2 and 10 speed transmission. Feels like driving over rumble strips at low rpm(1,000-1,500). Truck has 52,000 miles on it. Took it to my local dealer and they said there was no problem and I was likely feeling the Dynamic Fuel Management. 2 weeks later it spun a main bearing and had to be towed to a different dealer and ended up needing a new engine. The service manager at that dealer told me the fix for the "rumble strip" feeling is to drain and flush the transmission, replace filter, fill with fresh fluid, and re program the transmission. He said he sees it all the time and it costs $800. My question is why would it cost $800 to fix the issue when the truck is still under the power train warranty?