If only that was on the DS, you could make a custom stainless steel chainstay protector and braze it into place and nobody would be the wiser.
I'm with @
Steelman54 that it may well have been a manufacturing defect. It almost looks like the paint curls into the seam (unless it has been repainted at some point).
The bike may well ride fine. You might look to see how the seam behaves when you put weight on the bike.