I picked up my 2020 Denali on 8/30/20, its a 8/20 build. The first day I got it, it leaked from the passenger side rear window, onto the plastic you talk about, and down the window. I am 100% convinced that most of these power sliders leak on either side. I had a 2019 Silverado prior, but it had the dark headliner, so it wouldn't have been as easy to notice. This truck has the lighter color headliner, which is what made me notice the leak first, the wet headliner.
I took it in to the dealer, and they said "oh yeah" one of these. Not very comforting lol
Somewhat mutually I decided I didn't want them to tear apart the headliner, spoiler, rear window, caulk etc to basically not fix the leak for good. I think these repairs just put Band-Aids on if anything. It would be nice for the 2022 redesign to have a permanent fix for this.
I have had to have my truck towed, due to a battery that lost all its cranking amps (Wouldn't charge at all). I also am in the build date range for the valve spring issue, hope that doesn't happen!!
Good luck with your dealer.