Chosen Solution
I just bought the new iPad and I’ve noticed (when it’s off, during condition checking) that it has a line right ar the middle of the screen. It doesn’t make any difference in the brightness, colour or sensitivity so it might be something like a “seam”. In any case, I just need somebody to say that it’s “OK” so my wife can relax. :)
Did you buy it new or used? Technically there shouldn’t be any seams so I can’t tell you why this one has it. If its used its possible it was repaired and the marking was caused during the repair.