Hey there,
luckily my screen doesn’t look like that (so far) but I did disamble a few FP3 and this is my observation about heavily used screens like in the picture:
The front glas of the display unit holds to the frame of the screen with a black, flexible adhesive band. If the glue from that band doesn’t hold the pieces together anymore, these narrow, straight (mostly horizontal) lines get visible.
Initially this is only a cosmetic problem but can lead to the screen itself getting lose from the frame on one corner or on one side. Once that happens the screen is obviously more sensitive to humidity, dust can also gather around. If a noticeable gap is visible, I would recommend to glue it with T7000 glue which is black and mostly comes in tubes with a really thin opening.
Best regards