My new Pixel 5a 5G has this discoloration on the back as well. A vertical line running from top to bottom on the right side, and two small circles about 1/4" in diameter just under the camera bump. Seems like defects in the coating as water "wets" the surface differently in these areas. The two small circles are probably fixturing marks.Only used it for a couple of days. The coating on the right side is already worn out. There is no damage at all (any kinds).
I have contacted Google though Chat on the website. I was told "this is cosmetic damage, it shouldn't affect the device functionality". They will not issue a replacement.
This is probably my last Google purchase for a while. Do your Pixel 5as have the similar problem?