I had installed an OEM screen on my 5s, replaced the buzzer and put a new battery in. Had the same thing - "faint squares on the top of the screen" especially visible in passcode and locked screens.
The rest of the LCD looked great.
Took the screen off again. Made sure everything is connected properly.
Put the screen back on - same story. Had a look for possible clues and noticed that the body is all "wavy" due to tension somewhere.
Took the screen off again. Removed the shield plate all together above the clips. Made sure that everything is clipped in 110% as the board was mushy and had signs of dampness or water damage. Loosened the screen assembly screws.
Popped the assembly back in.
The shadowy squares at the top have disappeared.
Everything works well
So inn my case it wasn't the cheap Backlight / LCD layers (although i did drop it on the tile floor from a decent height) but RATHER the dodgy logic board connectors and the chassis flaws
p.s. Long story short - do only those 5s's only if they have NOT been opened before.
Otherwise think twice who tried to repair it and what was the history of that handset