Does your button feel like it's covered in silicone on the inside?
First of all, thank you for completely triggering a full OCD phone and case clean down. It set me back 30 minutes of cleaning and ensuring perfection, lol. Unfortunately, I do have actual OCD, but I make the best of it, hah.
Anyway, when doing said full clean down, I inspected the case even more (LED flashlight mode) to find signs of a residue or sticky substance. Nada. On the inside of my CC button, it feels like it is surrounded by a rubber flap. If I used compressed air on that button, I can see the rubber flap move a tad. I've included a photo below. It's actually one of the cleanest buttons besides Apple Sil that I've tried.
The CC button outside is just shiny. It's not matte, so it does attract a small amount of fingerprints...but I'm not really caring about that.
I'm really frustrated for you because this case (mine anyway) is actually pretty damn solid.
(When using the flashlight, I was able to notice a very slight indent on the back from the magsafe ring. But yeah, flashlight..haha)