Originally posted by clackers The OP's claim is the RAW files looked like that straight from the camera - no corrections.
Even with JPEG it is very very unlikely that you would have noticeable banding on the sky, because normally there are about 40 steps within a stop in the middle 5-6 stops of the image
To get the type of banding here you need to cut that down to about 6 steps per stop, which means you are working well out of the middle +/-3 stops of dynamic range.
I suspect there has been some form of either SCurve correction or mid tone range expansion done. Maybe an auto exposure application upon generating thumbnails