It may have been that the SN wasn’t stamped at the factory at the right point so they stripped the paint and stamped it, forgetting to paint over it.
If you have paint and can make it good then that would be the simplest option.
I know you want to understand who and why, so that you have confidence buying again, but it’s a bit of a weird one.
You might feel like it’s a bit crap, but as others have said, the decision is what do you want, is it still acceptable? You may or may not get more of a discount off Wiggle but if it was £25 less and the defect was pointed out, would you have still bought it? If you return it, are you going to get as good a deal elsewhere?
FWIW, I’d paint over it, helitape it (the BB and downtube get a battering from stones!, my BB chipped though the paint all over before I helitaped it) and get riding. Life is too short to chase the pennies sometimes.
and welcome to the forum!