(Pal og 3ds) Why is there a black line on my top screen that changes size even if i do nothing

I bought a used 3ds a few weeks ago and changed the top screen lens but a few hours later a little black line appeared( photo 1). I thought i just messed up while replacing the screen but 3 days later it was a full line( photo 2). I thought it just got worst but today its not a full line anymore but only a little one in the middle of the screen. So could i solve this issue without replacing the top screen ?