The Reddit thread dedicated to the issue has turned into the oh-so-rare quality discussion on why screens are prone to do that after a self-proclaimed Apple phone display engineer entered the fray.
While he tipped the audience on some known facts - namely that OLED screens bend towards the bottom to fuse with the connector that drives their pixels. This fragile bond is extremely prone to having traces that run screen lines detach upon bottom impact with the ground, especially if the chin is thin. The pink lines that form are the result of said broken traces leaving pixels they command permanently on. The phenomenon even has a name in display engineer lingo - a VLD (vertical line defect).
Given that this 180-degree bottom bend is done on most OLED panels with plastic substrates like the ones most flagships are now using, everyone's OLED models are prone to VLDs, especially if they have thinner "chins," i.e. bottom bezels. When asked why isn't every OLED phone made with thick chins then, the purported Apple display engineer dished out the following fascinating answer:
There you have it - Apple loves aesthetics and is willing to sacrifice some fragility for thinner bezels (no worries, the iPhone XR is safe), Samsung, on the other hand, as the only one among the top smartphone makers that don't have a notch-y phone, prefers a frontal symmetry making Notes and Galaxies less prone to VLDs.
Such Samsung design concept may even be brought to the extremes when the Galaxy S10 lands, as rumors are swirling that it will have even thinner top and bottom bezels than the S9, achieved by tucking the front camera within a tiny hole in the display itself instead of the top bezel.
source: Kymbb (Reddit)