Interesting, I just RMA'd mine because of this and they asked about liquid damage, etc.. I had no idea this was common. It almost looked as though some purple ink got under the screen.
That's oxidation in the OLED cell. Normally there's supposed to be a hermetic seal around the display to prevent this, but manufacturers have to balance quality and durability against cost and bulk. Samsung just didn't quite hit the mark in that generation.
Samsung have made AMOLED for smartphones for about 10 years and such issue isn't very common on most phones. I doubt Samsung doesn't failed but phone manufacturer failed.