Small row of "blue stripes" on top of iMac 27" screen

Recently returned my iMac for a newer one (previous had a dent in the casing…).

First time I saw this blue line was during the shutdown gray screen, which led me to believe it was the screen acting funny on shutdown. Turns out, it's still present, though only appears when there's grey or dark screens.

Here's a picture (hard to see because of the distortion lines from the camera). It's right at the bottom of the grey part of the screen (I've highlighted one side to help locate it).

Different angle (it doesn't seem to go all the way to the edge of the screen):

Does anyone know what might be causing this? Should I have it exchanged (still within the 14 days period).