I am having the same issue with my watch purchased in April. Took to Apple Store and was told it is not covered by warranty by a technician. Then a supervisor came over with a magnifying glass and looked me in the eye and told me I scratched it off with the charger or with dirt?!! So why is the glass at the front or the back not scratched? No response.
These are not scratches, these are circular pitting that seems to be eroding the back of the watch, not scratches at all. The sides /wheel where mostly handled has no scratches or pitting -as yet, so why on the underside??
I wished I had looked at the forum beforehand and saw these examples, instead of being gaslit by the store when they must have seen this on numerous occasions. It is extremely disappointing that an item designed to be worn 24/7 is degrading underneath where it should have the least impact. Is seems it not designed to be near human skin only sit on the charger. I was also told it was because it was an aluminium case- oh dear my fault, I should have spent more money on a better watch!!!
So I did not get a positive result from going to the store. Very upset and disappointed with Apple and the service I received. 

