This looks like the clear protective film that the doorbells are shipped with and that is intended to be removed when installing the doorbells. But it also looks a lot like other customer reports (going back years) that can be found when searching for "peeling Nest doorbell". If it's not the removable protective film, then it seems like a product defect. Our Google Nest doorbells are 29 months old and 12 months old; each gets at least some direct sun part of the day, and each looks like new.
Perhaps you can reach someone in support via this link: https://support.google.com/googlenest/gethelp